Your place to discuss your deck ideas, find MTG resources, solicit feedback, and maybe even find some local players without going to Friday Night Magic (FNM).
GWJ has a sealed league in MTG:O. You can find all the details about that in the Sealed MTG:O thread.
Here are a few useful resources I have found for my deck building:
Basic MTG Rules - Everything you need to get started. This is particularly pertinent if you're starting up again after not having played for years, the basic game structure has changed.
tappedout.net - Great for prototyping new decks. You can draw sample hands, see your odds of drawing a card, view your mana cost breakdown and your average cost for the deck, and even set up a trade binder if you want to trade with other folks.
Gatherer - Everyone probably knows this one, great for finding cards. Not so great if what you want is "things that gain you life", but great if you have a keyword you want (Exalted, Lifelink, Deathtouch, etc.) or if you want to get a quick list of something (i.e. all Red cards which cost less than 5 from Innistrad). Also has all rulings for cards, in case you need a question answered.
MTG Salvation - Great site for articles, threads on deck building styles, spoilers for upcoming expansions, and up-to-date card errata.
Magiccards.info - Great site for card pricing, it pulls prices from all the popular online retailers and gives you a nice table for comparing. Also has a very useful proxy printing feature if your house rules allow proxies, set legality list, and rulings on the cards.
MagicWorkStation - Another deck building site, and has a client you can use to play with folks online.
EDIT: Now with more thread hero image!