Well, unless you want to code it yourself (which is entirely possible, I would do it if I had the time), you will want to look into some kind of game designing program.
Since you specifically mention RPG, you'll want to look at the RPG Maker series:
RPG Maker XP (29.99 USD):
http://www.rpgmakerweb.com/product.html?prod=xp
RPG Maker VX (59.99 USD):
http://www.rpgmakerweb.com/product.html?prod=vx
For the record, I use RPG Maker VX. Kleptomaniac used XP to make Cackling Cave. Both come with a free 30 day trial, but that's hardly enough time to really get to know the program and actually figure out how to use it. I started with XP, made a few games, then quit for a while and just came back to VX for Tower of Gargalesis. My programming classes have certainly helped, though they aren't necessary to be able to do some of the simpler stuff.
It's a very intuitive program, but you'll definitely need to experiment/research to use either one to its full potential.
You can try to look for RM2k3, which shouldn't cost money, but I find it the hardest to customize due to the battle system (though you might like it better since it's a side view system).
If you need help, try looking around:
http://www.rpgmakervx.net/index.php?
I check the forums daily and have made a few posts. Reading around, you can learn some really cool techniques and even find free scripts people have devised that can really alter the gameplay.
Have fun with it, but don't expect to push out something awesome in a short span of time. I've been working on Tower of Gargalesis since January and it's only around 25% done :\