Xbox 360 gamers, I need assistance?

I used to play COD on a PC but I recently went to my mate's house (who has an Xbox) and I decided to give it a try. In conclusion, Nazi Zombies was a lot easier on an Xbox but when it comes to online, I really really suck. Since you still have to aim and by the time you've got a perfect aim someone shoots you and it takes me a lot of shots to kill a person while it takes them probably 2 shots to kill me. Even though I really suck when it comes to online, I am thinking on buying an Xbox. But do you have any tips how I can improve my shooting skills on an Xbox.