I use In for like saying that i'm inside something like
I'm "in" a car, or i'm "in" a house.
I use On for if i'm on top of something or at the top of something like
I'm "on" top of a horse, or i'm "on" the top of a Skyscraper.
I use: At for when i'm at a place like
I'm "at" a pizza place. or I'm "at" a lake.
In can also be said if you visit a country like: I'm "in" China.