"To aim at" is to target someone or somthing, usually with a weapon or some other projectile, such as a baseball, for example. "To aim to" is an in formal way of saying "to intend to" do something....
AIM is free to all users, but first you have to download the software (It's easy). Then get a screenname. Getting an AIM only asks for an e-mail and some personal info like your name.