1. Make sure your FPS is at least a steady 40, or you won't be able to get any clean kills
2. Don't spray. You should burst fire or 1-tap
3. Learn to peek (I would suggest TDM practice on Ship)
4. Join HS only games and learn to go straight for the head
5. Don't get into S&D Games until you're pretty confident
6. Get the weapon that you would be the most comfortable with (AK for power-play, M4 for stand-offs and versatility)

That's about it