On entering the USA you must clear your luggage (checked and carry-on) through US Customs at the first point of arrival. In your case this is MIA.
After clearing immigration you need to pick up your checked luggage from the carousel and then proceed towards the exit. You give the white customs form to the officer who will either smile, and waive you through - or frown and send you to secondary where they will search your bags.
In both cases there should be AA's luggage counter immediately BEFORE or AFTER (I don't know MIA that well) you exit the room (as mentioned by Mr. @ Spotty M above). It should be clearly marked for connections. Queue up and hand your luggage to them. You will need to shown them your boarding card (or your e-ticket).
Then head for the security, and the connecting flight's gate.