"I am all for it" means you are strongly in favor of, or completely agree with something.
I don't think I've come across "I am all about it". Do you mean "I'm all over it" or "I'm all that"?
"I'm all over it" means that you've taken on the responsibility of performing a particular task. (Different from "I'm all over that" which means you no longer have strong feelings for a particular person or thing).
"I'm all that" is usually used as a boast. It's not a common slang term.