I'm American. Willing to study in USA. Should I do IELTS? Will it help in any way? (I.E. credits).?

IELTS and TOEFL are tests of English proficiency, for non-English speakers. I interpret your "I'm American" as saying that doesn't apply to you (although I suppose you could mean you're merely a US citizen but have never lived in the country or even spoken a word of English in your life - in which case, perhaps you could have mentioned that?!).

And no, you don't get any 'credits' for being fluent in English, that's pretty much taken as read.

IELTS is a test of English language ability - British English, not American English. If you are an American raised in US or English-speaking country, you do not need TOEFL. If you are US citizen raised in non-English speaking country, you must take TOEFL! No credits. It's required to apply for admission.