It's one of the hardest parts to learn, for sure. There's some sage advice here:
, and I would just add that when it comes to prepositions the basics in my opinion are best learned from school books, specifically by doing the exercises within. Luckily most of the time prepositions like "in" (inside) or "on" (on top of sth.) are used exactly the way you'd expect them to be used, so you'll get the hang of it in no time.
For all the exceptions, set phrases and special cases it's just practicing the language and they'll come naturally to you. You can try to memorize structures that you need to use repeatedly/often. For example, if you travel a lot and need to be able to discuss about it, you'd learn sentences like "I'm flying TO...", "I'm staying AT...", "I'm leaving FROM...", "I need to leave AT..." etc.
“On the force” — I’m afraid this is idiomatic. It’s comparable to “on the team.” Guess what — in British English , they say “in the team.” It wouldn’t surprise me if they also said “in the force” — actually, while I can’t recall ever hearing that in American English, it would not be wrong, according to the general rules (in the army, etc.).