They're two completely different words; "to" is a preposition of movement (I am going to Mexico, I give it to him) which has loads of different uses that you can look up in a grammar book or website. It is also an indicator of the infinite of a verb ('to eat', 'to drink', 'to err is human'), "For" indicates intention towards rather than movement: "It's for us", "What did you go there for?" It would take me an hour to explain the difference in general between the two of them as they're so different.

if you mean as a preposition after verbs it depends on the verb and the meaning, different idioms