The best I can do is to give you a definition for the the word preposition and you can take it from there.
"in grammar, a word which is used before a noun, a noun phrase or a pronoun, connecting it to another word"
Each of the prepositions you mentioned:
"to, for, in, on, of, at"
all have numerous purposes.
For example for:
#1: as a result of something
#2: having the purpose of
#3: Intended to be given to
#4: used to show an amount of time and distance...
and so on.
It would take a really long time to explain the functions of each preposition.
I'll Google some more to see if I can find something...
EDIT: Ok I've found a website which provides a lot of information. Click here.
For the record, in case you're wondering which prepositions to teach, according to this website:
"Just an aside: The prepositions OFF, TO, and IN are among the ten most frequently used words in the English language."
Still according to the same website, the most frequently used prepositions:
I thought this could be useful in case you're wondering which one you'd like to teach.
Student or Learner