Look sensitivity depends on the player. Mine is really low while others have it very high. Like with anything in life all it takes is practice. Don't be obsessed with winning, just play to get better. Start by practicing with things like campaign or the strike, and then go into crucible. The first will help you learn the mechanics, and the second will sharpen them. And, most importantly of all, don't let sucking bother you. I suck soooo bad, but I still have fun.