If you're interested in D&D there are many resources out there for you.
For the current edition wizards have this great little page to get you started.
http://www.wizards.com/dnd/TryDnD.aspx(the site is an absolute nightmare to navigate, don't get lost now)
If you'd like to play a little 3.5, the previous edition, a lot of the rules are made available free via the Open Gaming Liscence.
http://www.d20srd.org/(This site is agreed to be one of the better ones)
I believe other games have these little taster things, GURPs lite is another example of nice free stuff to get you hooked. When I started playing RPGs, honestly not too long ago, I joined a couple of play-by-post games on a forum I had joined. Luckily enough I managed to get a great GM for one of the games a dreadful one for another. This contrast made me realise one thing; learning and running a game is tough but if you do it right everyone has fun. So about a month or two after getting comfortable with the rules for 3.5 I ran a PbP game. It has been going for nearly a year now and I am glad I did it because it segwayed me into playing around a table (an entirely different experience I assure you).
I know I probably shouldn't be posting links to other websites for pure shout-out but the forum I did all this on was the one for the internet reviewer/comic/critic Spoony. His forum has a very active PbP section so if you don't find luck here you may want to try there as another option.
www.thespoonyexperiment.com
