“I’m in” means that you are inside something. In contrast, “i’m on” means that you are on top of something. For example: I’m in the car. (Meaning you are inside the car. If you said “i’m on the car” it means you are on top of the car) I’m on the bike. (In this case it wouldn’t make sense to say “i’m in the bike” because that would mean you are inside the bike 😅 so by saying “i’m on the bike” it means you are sitting on top of the bike) I hope that helps.

It depends on the situation. Unfortunately, in many languages the prepositions need to be memorized because the logic is not intuitive. Good luck!