Healing a raid depends on what you're doing. As a Druid, you'll very likely be main tank healing. This means one thing: Treeform and drop lifeblooms. You'll want to keep a full stack rolling on the tank at all times. However, if you're not involved in that, then you'll be dropping HoTs on the raid as they need em.

Go run AV. Run to the Horde's relief hut and then assault their general. Now you'll have a tank (whose identity is generally a complete mystery pre-fight) to heal as well as a wandering tribe of dillholes scattered about 8 different raid groups. Do this until it eventually makes a wee bit of sense, then you'll be more than set for raids.

Healing with a Resto Druid is not very hard. Your biggest issue is that almost all your heals are heal over time. Best thing to do is always keep lifebloom x 3 up and one of the heals you can pair with swiftmend. I usually pull the tanks and the other healers out separately. If you can keep those folks up, then the rest is easy.

As far as the numbers go, lifebloom will credit the person you healed, not you. It is very frustrating to be last on the healing list and not be able to figure out way.

The HoT registers with the druid, the bloom does not. That's why its rather odd. But yeah we have a leafy druid in our raids, usually 2, and they are definitely up there on the meters.

I've done that many times.

An extra heal is certainly better than a missed one.

Healbot is also pretty useful for showing who the other healers in the group are currently targeting with a heal, though it AFAIK doesn't readily display other healers HoTs, so the druids are generally a mystery.

Other questions I have are regarding setting up Xperl for raids. I messed around with it this evening. Any advice on how I should set it up?

You might want to also use a wisdom elixir with your healing power. You'll do fine, and you'll get the hang of working with other healers through practice. Follow the tips already given and you're golden. I also like Healbot. And overhealing is nothing to be afraid of, unless you are mana starved.

As for crosshealing, I keep an eye on the entire raid unless I'm strictly assigned to a tank on a boss fight, and even then if I can. Healbot is awesome for that.

Edit: Have fun!

Yes. I was playing around outside an instance and the message was Dyrrian receives x from Dyrrians lifebloom.

I'll chime in on a few things since I've gotten much better and more confident in my healing abilities over the last several months despite contradicting some of the advice here.

Healbot/Decursive - I tried Healbot for a few weeks, and I found that in general, it actually was slightly less responsive than doing the healing manually. Now, the big caveat here is that I'm a paladin healer, so I really only have a few abilities to do any healing, and of those only two that get used frequently: Flash Heal and Holy Light (i.e. Uzi Heal and Bazooka Heal). Holy Shock is also good for an insta-heal, but Flash Heal is really darn fast and heals for more, so as my +healing and mana pool have increased, I find that it's easier just to overheal a bit with Flash Heal than depend upon Holy Shock to save situations.

I'm digressing. My point is that Healbot, given that I really only need to worry about three shortcuts on a frequent basis, was less responsive for me than doing it manually. This is totally personal preference though, try both ways and see what's more comfortable and responsive for you. For some people, that's Healbot. For others, it's not. You need to run several raids/groups with each method to get a good feel for what you prefer.

Crosshealing - Short answer? As Tundra said, an extra heal is better than a missed one. From what I've read and experienced, you're going to experience anywhere from 15-25% overhealing depending on the specific encounters you're running and the raid composition. Some classes will overheal more than others, too (HoT's will tend to overheal more than targetted heals in many cases from what I've seen, but that's just fine!).

Healbot indicates people that are receiving healing from other members of the raid, but the latest versions of X-Perl (my customizable UI of choice) do too. Members receiving heals get a green highlight on their background & border, so I can prioritize members who aren't receiving heals currently. It works nicely.

Generally, depending on the encounters, either the raid leader will assign healers (on fights where tanks receiving consistent, constant healing is critical) or the healers will simply organize themselves. For raids where the healer team is pretty experienced with each other, we often won't even need to organize except in a few spots, because we're used to how each other works. For instance, I've done raid healing with Kiri and Juturna so much that we only need to worry about picking targets in very intense fights. That's something that'll just come with time, and it took me a long time to get to that point of comfort!

Don't worry too much about crosshealing at first. It's better (especially when you're learning raid healing) that everyone be topped off as much as possible than it is to worry overmuch about efficiency. The efficiency doesn't get as crucial unless you're dealing with a starved mana pool or you're running raids that have a timer (like ZA).

General Tips - Just some general stuff that I can think of to help with suggestions.

If you're using X-Perl or another custom UI with similar features (and I HIGHLY recommend this as a fellow healer!) there are some settings that'll help you out. Showing health deficit rather than totals is a good start: instead of showing how much HP your group or raid members currently have, it shows how much HP they're missing (but the bar still shows health the normal way if you want). This is great, as it tells me two things at a glance: which heal would be more appropriate for them, and how hard/fast they're getting hit. If they're taking damage reasonably slowly and have a lot of health missing, I can throw a big heal their way. Otherwise, they get a quick Flash Heal if they only need a little healing, or I turn on the Flash Heal uzi including popping some trinkets/cooldowns if they're getting hit hard and fast.

Another great option I love in X-Perl is that, like highlighting targets getting heals, it highlights targets getting aggro and getting injured. People with a red highlight are getting aggro, so I can prep heals for them as needed. I see someone flash red who's not a tank, I start firing off a flash heal their way if I can, because they're about to get slapped by something!

In general, don't let tanks get below half health. Doesn't matter if they're "your" tank or not; if a tank's hitting half health or less, and you aren't needed elsewhere, starting firing heals at that tank asap.

Heal the other healers asap. Even if they're only a little injured, it's important for two reasons:
1) healers dropping is bad, so keep their health topped off as much as possible to reduce the risk of that
2) when healers have been injured, it may have been due to aggro (you never know), so it's better for another healer to help them rather than for them to compound the situation by healing themselves.

Heal everyone else. Yes, your priority is who you're assigned to heal, but never restrict yourself to just that. Heal anyone who needs it whenever you can spare the time & mana.

Lastly, learn all your cooldowns and trinkets well, so you can take best advantage of them. Also, most trinkets have short cooldown timers, and can be used multiple times per battle. Don't leave them unused, thinking "what if I need it later?!" You know what? If you use them frequently and effectively, there's much less chance you'll need them later. For example, I have one ability and one trinket that both help with mana regen and efficiency. I use them pretty early in each fight to help sustain my mana pool. I use them again as often as I can to keep extending my mana pool. Works great, and I chug a lot fewer potions as a result!

That's all I've got for the moment, I'm sure I'll think of more stuff later. I don't know a lot about druid healing or I'd give you tips more specific to your character, but I've played a healer in just about every mmo or team game I've played (yes, I played a medic in Tribes 2 and play one frequently now in TF2 as well!). I love the role of healer, for whatever reason it's been my favorite all along!

I'm interested on how it went in Kara? Were you successful?

I hope so, lol.

As the healing lead for my guild, I'll offer some insights. I've run our crew from Kara, ZA, SSC, TK and into BT.

Mods:
Grid (wowace.com) - I find this to be a great wealth of information for raids. You can see aggro, health, incoming heals and debuffs. It is compact but very dense with information. Great for everyone really, not just healers.

HealOrganizer (wowace.com) - In terms of defining who is healing what, this is a great little mod to use. You can broadcast healing assignments in a healer channel or whisper assignments directly.

I gave up on Decursive, as Grid gives me enough information regarding dispelling debuffs.

I have no experience with HealBot.

Cross-Healing/Healing Assignments:
In a full on raid setting, I generally discourage cross-healing as a norm. Of course, in an emergency it is perfectly acceptable. I organize my raids in the following fashion: burst healers (paladins and holy priests) on tanks, one dedicated druid to keep HoTs on ALL tanks, and druids/CoH priests/resto shammies to heal the raid, by group. The reason I try and ask my healers not to cross heal is that I can be a better judge of where healing resources should go. If healer A is doing his job and spotting in healer B's groups... it will be harder for me to figure out that healer B is having issues. Additionally, healer A is wasting mana for his own group assignment. In any case, the specific focus of healing assignments generally makes it easier to heal the raid.

Consumables:
Find someone who has the Mana Potion Injector Recipe. Mana potions stack to 5, the injector stacks them to 20. I carry 60-80 mana potions on me at all times across 3 or 4 injectors.

I carry 300 Symbol of Kings and 5 Divine Symbols for DI.

Also, I have 20 Golden Fish Sticks (+healing) and 20 Sporefish Filet (+mp5) for raid food.

I used the Draenic Wisdom and Healing Power elixir combo for a long time. However, flasks are better and cheaper in the long run. They last through death. If you are wiping while learning new content, elixirs can get expensive. Your mileage may vary.

Basically, stock up.

Let me know if you have any specific questions, I'll be happy to help.

Whether or not flasking is cost-effective depends on your server costs and your elixir choices. With the elixirs Zel uses (major agility and major fort), she has to die about ten times to pay for a flask. Dying ten times in two hours is pretty improbable, in my experience, anywhere but a BG.