As others have said, stick with your teammates. Having multiple players focus on one target makes short work of it, and when you start getting shot, your teammate can provide covering fire for you. Once you become more experienced, you can start to go solo, which relies a lot on accuracy and reaction time. Halo is whole different style of gameplay than cod, all it takes is some time to adapt. Also, on maps with vehicles, try go be in a position where you can take cover if you come under heavy vehicle fire. If you get stuck out in the open with an enemy vehicle, consider yourself screwed