Headshots, Playing Corners, Playing the Map, Playing the Players, Playing the Game
Fairly straightforward, aim for the head, as fast as you can. Find your own headshot rhythm with your favored gun. Headshot Rhythm: a good spacing between shots that allows for the quickest headshot successions and recovery from recoil.
Do your best to engage enemies while you are near a corner or other obstacle that will allow for quick strafing in and out of their field of vision.
Pro-Tip: If you are in the red, don't keep strafing. Live to fight another day.
Playing the Map
Know your environment. Know the best places in the map to engage that will play to YOUR strengths (ie. Don't rush into hallways or corners if you don't have a shotgun). Know where the objectives are, and the best places for you to have the advantage when engaged near those objectives.
Playing the Players
Watch for the most common ways that other players counter you, and try to learn from it. Keep an eye on where your team is (approximately), and what you are doing to help your team progress. Watch for what the other team is doing and communicate to your team, especially if you are outnumbered.
Playing the Game
HAVE FUN! It's a game, and if you don't enjoy it, don't play it. Know yourself, and your limits. The game can get frustrating, but at the end of the day it's still a game, don't allow the character and space you control to control you.