One of the best places to start is the How to get your rivers in the right place sticky in the tutorials section of the forums. Personally I like to place my rivers first, and then place mountain ranges and hilly areas in the high places the rivers defined.

Also, if you want to get really in-depth into climate, there are several climate-building threads in the World/Regional WIP forum to check out; give them a perusal

And, as always, we're available if you have more specific questions, just post em up here.