Pathfinder is one of the most common systems, but it's not super-accessible for the newly initiated. DawnForgedCast does a decent series of tutorial videos, starting with :  <a href="http://www.youtube.com/watch?v=77nb34DB6Gs" rel="nofollow">http://www.youtube.com/watch?v=77nb34DB6Gs</a>   Paizo (provider of Pathfinder materials) also provides the ruleset openly:  <a href="http://www.d20pfsrd.com/" rel="nofollow">http://www.d20pfsrd.com/</a>  Actual Dungeons and Dragons is more closed source, but materials are out there as well if you look hard enough.  Keep an eye on the Looking for Group forum.  Very frequently you will find GM's offering tutorial or 'newbie' games specifically to help new people get into the genre and learn how to play one system or another.  You'll find the Roll20 community in particular is extremely helpful and open to answering any questions (though more specific questions are easier to answer).