First, welcome to the hobby.
Second, here are a few links that can allow you to take a look at the rules for some Dungeons & Dragons and some derivatives.
Pathfinder Roleplaying Game Reference Document (This is the Pathfinder reference document based on the complete manual published by Paizo).
The Hypertext d20 SRD (v3.5 d20 System Reference Document) :: d20srd.org (This is the Dungeons & Dragons 3.5 edition reference document also based on the complete manuals published by Wizards of the Coast).
Both of these are very similar in rules content and definitions, especially as Pathfinder is an evolution of the 3.5 edition.
Since your are a novice, you might want to check out a simplified rule set. Paizo just put out an introduction package. The Beginner's Box is available as a boxed set (a documents and material in a boxed package). However, the printed material is also available as a pdf file on the Paizo web site (
http://paizo.com/beginnerbox).
There was a similar product put out by Wizard's of the Coast for the 4th edition of D&D. As I did not make the switch to that edition, I am less familiar with the products. Someone around here is bound to be able to answer you.
If you are more into modern settings, there was a product, now out of print, called d20 Modern which used similar rules as D&D 3.5. You can find the reference documents here:
SRD Home.
If Superheroes is more your style, you can check out Mutants & Masterminds from Green Ronin (
Mutants & Masterminds Headquarters -- MutantsAndMasterminds.com). It is also based on D&D 3.5, though it has been heavily modified.
This is by no means an exhaustive list of role-playing game systems. I limited my self to D&D and some of its derivatives. Other systems include but are not limited to: Savage Worlds, GURPS, Warhammer, Dragon Age, Hero, Storyteller.
Good luck,