I want to differ slightly with Andrew: in the USA, at least, you would never hear someone say "I agreed for help" or "I am agreed for help." These are not accepted usages and sound very unnatural.
"I agreed to help" is correct, meaning you were willing to give help. The opposite situation requires more. If someone offered to help you and you accepted it, you might say, "I agreed to accept help" or "I agreed to accept his help" or even "I agreed to accept his offer of help". I hope this helps!