There’s so much D&D stuff available on the net that it’s possible to create your own games from scratch.
First I would recommend that you check the D20 rules. That’s basically the D&D system stripped of specific setting elements. It was made available for free so people could make their own games, and they did, everything up to and including superhero games. From that you can learn the basics, such as what things like “Hit Dice” or “Armor Class” mean.
Once you understand that stuff, just google up D&D online (there’s even a D&D Wiki) and you’ll find (very detailed) descriptions of character classes, monsters, magic items, etc. Enough for any homebrewed campaign. Or you can buy a module, those come with everything except the rules, and you can run a ready-made campaign. Again, you can order these online or buy them at a bookstore like Border’s (I think Borders also takes orders.)
As for other tabletop RPGs:
-GURPS, the Generic Universal Role Playing System, has pretty simple rules (their books are big more for the long lists of skills, etc. than their complexity.) They have books that offer adaptations of licensed stuff, including some obscure ones (ever wanted to RP in the War of the Chtorr novels? Yep, they got it too!) Plenty of original stuff as well. Check them out at the Steve Jackson Games website, you can order directly from them.
-If you like superheroes, there’s CHAMPIONS; it’s “Hero system” can be used for nearly anything else. Probably the most comprehensive rule system of all, but also the most complicated.
-There’s plenty of other stuff; again, just google “tabletop RPG” and you’ll find it.