No. Autoaim isnt an option for online play; well unless you have a development kit, and want to spend some serious time hacking the code...
You have to select the RPG in the weapon select screen, then ensure you pick some ammo. So its a minimum of two equipment slots. In-game you must be stationary, select the RPG (so R2, scroll to RPG, R2) and the character will then kneel. You can NOT shoot when moving.
Last word - practice, practice, practice. I'm not a very good shot, but i engage people from the sides/rear after they have gone past, or are attacking my teammates, kills are much easier to come by then. Also you could try playing the single-player game with auto aim off, its a slightly easier time with no preassure, then go back online when you are feeling more confident.
