Getting rivers in approximately the right place is fairly easy. To do it automatically with a PC is pretty hard.

Firstly - take a look through this little tut. I wrote this but its actually a collection of ideas and stuff we have talked about on these boards for a while by many people.

http://www.cartographersguild.com/showthread.php?t=3822

I write an app which calculates rivers automatically which is the GeoTerSys link in my sig - theres some movies on there to show what its doing. But the app is one of several trying to solve this very hard problem. Its a LOT harder than it first appears. Obviously water flows downhill and we can program that easily but that's not the end of the issue. It gets complicated from there and thats where the real fight is taking place.

Waldronate (joe) on these boards writes Wilbur which is what Jezelf (also on these boards) was using to get his rivers out of his terrain. If you look in my sig again there is a keyword index. Look up Wilbur in there and Joe has done a few tutorials for it and shows how to do the rivers using it.

Apps like Wilbur, mine, some of the GIS apps and the odd one or two more are the only options in this area right now and since Wilbur is kindly downloadable for free then I would suggest that would be a good starting point.