What or that ?

Which is suitable for the blanks in the following sentences,that, who, whom, or what ? And should the word 'think' be followed by the word 'about ' or 'of' in sentence 2 and 4?

1. you think is what you get.

2. you think about is what you get.

3. Do they know the boy you think is your classmate?

4.Do they know the boy you think of is your classmate?

One more question:

Tom was reading a book, was about a poor girl, borrowed from the library. Should we put commas , , in the sentence, or without putting , , ? What is the difference?