I have no idea why people are saying body shots, when that is the worst way of learning. When firing at the body and you quickly go to aim at the head, you can either be too late in the aim or miss it entirely. You should always be aiming at the head with any single firing rifles/handguns.
As SEC Archangel and Zelda said above me, play SWAT, and play it a lot. It significantly improves you head shot aim and efficiency of shooting, that is, not wasting shots. When in voting, don't pick magnums, as that's a waste of a game, though that can help with head shots, but you want to play with the DMR not that.
When voting in matchmaking for a map, try picking either Countdown or Sword Base. Countdown has many close quarter areas that put you under the pump, which can make some players panic and not aim well, if you get this under control, aiming will not be a problem. As for sword base, there are many long sited areas, so that improves long range accuracy.
When you get your aim sorted out, you should be very well prepared for the other playlists, while being very good, and able to take down a lot of enemies.