For free! cockatrice.de
That being said these are useful links for learning how to play, last two are more about strategy while the first 3 cover the basics:
http://wizards.com/Magic/TCG/Resourc...?x=magic/rules
http://wizards.com/Magic/TCG/Resourc...ources/formats http://wizards.com/Magic/TCG/Resourc...sources/banned
http://magic.tcgplayer.com/
http://www.channelfireball.com/
@Wyrm: Winning with $1 rares. :trollface: