Hi Lucas! Good question. Generally, 'on' means on top of. For example, I put the book on the table.
'In' means inside of. For example, I put the book in the box.
However, there are exceptions: Even though you ride inside of a bus, you would say "I am on the bus". Likewise, you would say "I am on the subway". However, you would say "I am in the car" or "I am in the taxi".
Unfortunately, many uses must simply be learned from speaking with a native speaker.
I know prepositions are especially challenging in English. Here is a website that might help you notice some more differences between in and on: http://www.differencebetween.net/language/difference-between-in-and-on/ Most of this website is in English. I hope that's alright!
There are many exceptions in the English language. Like Mena said, you'd really have to listen to native speakers or read what native speakers right in order to discover all of these exceptions.
thank you so mach mina i Also benefited from you