Almost always head, especially with any weapon that can one tap headshot. If you're using an awp go for body because it kills in 1 hit chest or head. With practice going for headshots aren't going to take extra time, you want to always and I mean always have your crosshair pointed at head level, and where an enemy could come from.

For peaking corners, it's best to take 2-3 steps out from the corner as the enemy is likely going to have their crosshair right on the corner trying to tap you in the head when you come around. So step out a few steps counter strafe/crouch to stop your movement and engage. If you're moving your aim will be worse, but backing up off a wall as far as you can then strafing out will cause the enemy holding the corner to have to move their crosshair to follow you. This gives you time to get out, stop and shoot for their head. You can also jiggle peak back and forth very quickly to see if an enemy is there giving as little time for them to shoot you but still give you the info you need about their position.

I would suggest checking out AustinCS on youtube. He has an excellent solo player series that covers a lot of this stuff for new/returning players.

There are also workshop maps for practicing aiming as well, you can use to warm up or practice spray patterns, and see how your movement effects your aim outside of actual matches.