Lots of good threads on weathering recently, use the search function.However, to be less snotty and give you a start, here's a good thread on on weathering with craft paints.
And I think the best weathering is a combo of paint for the base and (then) powder/chalk...
What is the best approach for beginer weatherers? What kind of paint should I use? Does chalk weathering look more realistic?
Thanks
Colin