The collectible card game Magic: The Gathering has been around for over 20 years. One of the strengths of the game is a strong support community, through both official and unofficial channels. And it needs it- as new cards are routinely released through several new expansions each year, it can be overwhelming for a new player to know where to begin- over 14,000 cards have been made. This post is intended as a one-stop resource for players of all backgrounds.
Rules & Formats
Both basic and comprehensive rules can be found on the official page here. One of Magic's appeals is the ways the game can be played- the different formats. Those are discussed here.
Cards
To search for individual cards (or text on cards), use either the official card database (The Gatherer), Magiccards.info, or magidex. They all feature the latest card wording for optimal clarity or resolving rule disputes (for example, some cards used to have abilities that trigger when it "comes into play," but now the language is "enters the battlefield." It's the same for rules purposes, and these online resources help verify that when questions arise).
If you're interested in tournament play, make sure your deck doesn't include cards that will disqualify you. Some cards have proven so powerful over the years that they have been banned or restricted, and this official page has lists of such.
If you'd rather visually peruse large sets of cards efficiently, MythicSpoiler is great.
Deck Builders/Viewers/Articles
Deck building is a key part of Magic. This official site has some deck building principles, and my recommendations on it are here. One way to deck build is to look at what others have already constructed (called 'net-decking'), and the sites in this section have plenty of examples.
Building a deck is most efficiently done virtually. You can do this in a number of places. DeckStats is a great site which provides a number of stats about the deck(s) you enter. It's a great place to spot any big problems during the design phase (rather than discovering them during play).
MTGGoldfish is another place to build your own decks or view others. It also has articles about cards (or card combos) that are quite helpful when designing. MTGTop8 lists which decks are winning at the many tournaments and championships all over the world.
MTGSalvation is another site with articles, forums, and other resources, as is StarCityGames.
Card Collections
If you want to store your inventory online, you can do so at DeckStats or DeckBox. The latter allows you to make your collection public for trading opportunities.
This official website has printable checklists for many expansions, if you're old school and prefer to track these things by hand.
Prices & Purchasing
Of course, if you want to play, you have to pay. Cards can be obtained from many outlets. You can get large collections on eBay or elsewhere. For factory-sealed products (like booster boxes, packs, and starter sets), there are many options. I've used CoolStuffInc for years and been impressed by their customer service and flexibility. TrollAndToad is also a great place to buy. I'm told TCGPlayer is good, too, as it lists private sellers as well as stores, giving you lots of choices.
As Magic is a living game, card prices fluctuate over time. This is because new card releases can create synergies with older cards that vastly increase value. MTGStocks is a site to track prices and popularity.
Video Coverage
Yes, Magic is popular enough to have plenty of sites dedicated to steaming videos of tournaments and other events. MTGCoverage is one; twitch.tv another.