I found this answer at: http://able2know.org/topic/128-1
TO is used with verbs (e.g. the verb "to be").
It is also used in cases where a "transfer" happens.
I will give this book to you. (from me to you)
I will go to work. (from home to work)
I will talk to her. (information goes from me to her)
FOR is used in the following situations:
"for the benefit of" e.g. I will do that for you.
"purpose" e.g. This brush is for painting.
Those aren't rules written in stone, they are just general guidelines
Here is an example with both: Give this book to him but it is for his father.
Another: I will speak to her for you.