My advice: If you are painting expensive rolling stock, seek out high quality pricy paints. For most everything else, the cheapest of paints are just fine. Some specific advice:
1) For covering large surfaces like backgrounds, base coats on mountains and so on, check out latex paints from the "oops" counter at the big box home improvement stores. Exact color rarely matters, and can be easily adjusted.
2) For everyday painting of all kinds, go for the $2 craft acrylics from a craft store or WalMart. There are a zillion colors available. Start with pure white, black, green, yellow and everything with "umber" or "sienna" in its name. Don't be afraid to mix colors.
3) The exception that proves the rule: Go to the expensive oil paint section and splurge on a tube of a color called "Payne's Gray". I learned this from following the work of Troels Kirk, who is a professional painter and a master modeler. This is an incredibly useful and subtle color that somehow brings scenes to life.
4) For stains and weathering, you can't beat ink/alcohol mixtures. Artists inks are available in many colors--kind of pricy, but they go a long way.
Hope this helps.