There is a whole set of rules when to use in and when to use at. The most common examples for at: I'm at the shops, I'm at school (places) I'm at the concert (events) it's at the bottom And for in: I'm in hospital I'm in church it's in the box I'm in bed. You will get the hang of it with time, but now I suggest you put "in/at prepositions" in google and try to memorize a few rules :)