How do I get money in the Wasteland?

Exploring new areas will yield you lots of loot to sell. In the beginning try the caves, ag center and Rail Nomads camp, then Quartz & Needles.

The beginning of the game is fairly loot-light, but once you get any sort of armor you can reduce your usage of ammo and defeat most enemies using Hand-to-Hand fighting. This means that you will have lots more ammo to sell.

Once you are on to the middle and later areas you will be mostly swimming in loot and/or money. The hard part will be finding a place that sells the equipment you want (most of the best gear is found, not purchased).

Once you have few thousand that you don't need to spend, you can gamble (at the Rail nomads is best) with one character with the Gamble skill & high luck. Once you get to gamble skill 5+ you will come out ahead. If you end up getting gambling to a high enough level than you can exploit the skill-point allocation wraparound bug: you gain skill points for allocating high levels of a skill at a library if the skill would cost more than 255 points to allocate.