@acampos1: I'd strongly suggest to never learn prepositions out of context. In my opinion, the best way to learn them is together with phrases in which they are commonly used. If you search for prepositions on the Duden site, you'll always find some common examples of contexts in which they're used. In this way, you get a pretty good intuition about their meaning and when they're used: http://www.duden.de/rechtschreibung/am http://www.duden.de/rechtschreibung/bei

Unfortunately there is no straight forward answer. "Beim" can have a whole raft of meanings, ranging from "while" to "with the" and many others. "Am" is perhaps a bit more simple, generally meaning at/on the. But it can also mean other things like "about the". Point being, there is never a one to one correspondence between the prepositions of two different languages, so you must learn by experience.

"Bei dem" (bei is a dative preposition.) [like aus ausser mit nach seit von and zu.] an dem is am