Start by learning the layouts of the map, stay on the outside edges, learn the cover points from the outer sides of each map. As you get more familiar, gradually move to the centres of the maps, learning cover points. Be patient, you will die. Learn where and how you died. Watch the replays after matches to see the strategy your enemy used to kill you. Also, watch the top players on either team to see how they play, learn from them, look at the weapons they use and how the apply them in their tactics. Start on a comfortable sensitivity, I have a good K/D in halo 4 (same as my forum name) and my sensitivity was always 5. Story mode is a good target practice warm up, as you get better increase the difficulty. Also make friends. Playing pvp with just friends before you go online is an excellent warm up and a great way to sharpen your skills and build team cohesion. Get familiar with one weapon at a time, I recommend the BR or scout as they are the most versatile weapons. Most important thing is to have fun, you won't learn anything right if you're miserable, hope this helps