I assume 90% of MTG Forge users already know how to play Magic, while this may be true, I’m still ignoring the other 10%. The best way to learn Magic is to download the Windows Demo. It will show you how to play the game. Anatomy of a Magic Card and the Rulebook will help solidify your understanding after you get the basics down. Tactical Protocol talks about some basic strategy and the Magic Academy should be all the reading a newbie needs for a couple of months.
And for a nice overview of Magic: The Gathering, check out Wikipedia.
Windows Demo 85 MB
Anatomy of a Magic Card
Tactical Protocol – how to get the maximum usage out of your cards
Magic Academy – a set of 50 articles for the beginner, just read thorough a few of the articles and they should help out your game.
For questions about specific cards:
Faqs for all Magic Sets
Searchable Rules - If I have a question about a card, I look here first, although it doesn’t cover newer cards.