Im a 16 in slayer right now but its not really a challenge. It gets harder around the 30sWhat is your current skill level?
Play loads of MLG customs with people slightly above your skill level. Once you beat them consistently, find new people to play with. Repeat as necessary and jump into a Ranked playlist (TS or TD for an easy 50 when you're confident, and then try running with a friend or two in MLG as much as you can).
As someone else has said, try beating the campaign on legendary. This will give you a good feel for the controls. From here, I'd say practice in the social playlists, particularly social slayer, multi-team, and rumble pit.