I would recommend finding a copy of The Players’ Handbook, 4th Edition. It costs about $35 dollars if you get it from Barnes & Noble, cheaper if you’re a member. If you don’t want to buy it, you can probably find it online if you’re willing to spend time looking. I have the PDF version and its about 69 MB so it’s not a terrible download.
EDIT: PS. There are other games out there. I personally have yet to find one I enjoy more than DnD, but that doesn’t mean they’re not out there.
Double Edit: Also, if you play it cheap, it can cost next to nothing to play. My friends and I are poor college students, so we play on graph paper with online dice simulators and PDFs a friend gave me for free. If you do have money, though, and end up enjoying it enough to invest in it, there are some pretty neat accessories you can purchase/build.
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.
Mutants and Masterminds, particularly 2nd edition, is always a good start, as it only requires 1 dice to play. However, DnD 4th edition is a good start.
there is a quick start guide that can be downloaded for free from the Wizards Website. it’s 27 pages long, yes, but half of that is a selection of premade characters.
if I was able to use any of the online tabletop game programs efficiently, I’d walk you through a game (though, I can probably see if I can make a working Play by Post session). After all, you’d want to get to know what a game involves before spending upward of $30/£20 on a selection of books without being garunteed that you’ll like it. I was foolish enough to buy the books before playing it, I’m actually very pleased that I liked the game, or I’d be stuck with a load of heavy, expensive books I was never going to use