This is not a trivial question. I have this post on another website. Perhaps it will help a little. http://englishworldwide.ning.com/group/lee-s-world-of-edutainment/forum/topics/in-on-at-are-you-bedeviled-by-these-prepositions

"I have been studying my English on the internet..." Look again online. The difference is very clear.

I am IN room. The book is ON the table. I am AT your door.

In: –preposition used to indicate inclusion within space, a place, or limits example walking in the park. On: –preposition so as to be or remain supported by or suspended from or used to indicate time or occasion example Put your package down on the table; Hang your coat on the hook At: –preposition used to indicate a point or place occupied in space in, on, or near example to stand at the door; at the bottom of the barrel.