If, as a marine player, I decide to place an aim order on my turn, I would get a regular attack or a move of 4, plus I place the order. The actual aimed attack takes place on my next turn, where my will also get to do my regular actions. Is this correct?

Yes, after aiming you can make any attack an aimed attack. But it has to be a point where you can actually make an aimed attack. So if you choose to unload in the turn after obtaining the aim order, you can shoot the first attack regularly, and make the second attack an aimed attack. It's odd, but it's what the rules allow. However, the player has to announce before rolling the dice whether he's making a regular or an aimed attack.

What also works though, since you can get the order at any point during your turn if you do a ready action, is the following: You can do a ready action, aim before you attack, and then attack.

Some leadership cards make that really worthwhile, too - those that let you move after killing an enemy, or that let you move and shoot during a ready order.

Thanks for the quick answer. I just couldn't find it in the rules for some reason. Now, I really want to go play.

But stupid work keeps getting in my way.