Basically... you send them to do surveying work. Most people (including me) just send them first to some usless bodies to get experience.
The thing to remember is that there are two types of bodies when it comes to surveying: small and big. Small bodies will have very small (new) deposits but more importantly, it takes a lot of time to find anything there. Becouse of this it's almost impossible to train your geo team on asteroids and small moons. For that matter for me it's rare to find new deposits on asteroids even with 250+ skill team.
So pick relatively large bodies that doesn't have minerals on them. The idea here is to have few small deposits discovered there for training and then discover several deposits in places you're already mining. Good targets (if they don't have minerals) are: Pluto, Charon, Titania, Triton, Titan, Galilean Moons, Luna and Venus. Mars and Mercury should be surveyed last (even if they don't have minerals) as they can be terraformed, allowing for mining with normal (manned) mines.