Welcome to the forums!
First things first, firepower is kind of a waste. It is nice, but you don't learn to compensate for each weapon and learn to play it as easily as using one exclusively at first. If you are worried about losing because of reload, use Dexterity, a must have for many players. Also, learn to use the magnum well (get a friend and start playing some magnum wars in custom games). I started to get good with it in Reach, and it is a nice thing to be able to use. But dexterity typically will be better than learning the magnum.
Second, the grenade pick up perk is very nice. You begin to aquire more grenades and you always seem to be able to have one, a problem I have noticed with Halo 4 (that you always run out of grenades). Pick whatever perks fit your style, but dexterity is a must have.
Third, when you are spawning think about two things:
1. Spawn now or wait. Will instant spawning be better for me to help my team, or will waiting to spawn because they are going to die/fall back be better.
2. When you spawn, press switch weapons twice. When you first switch to or pick up a weapon, there is an extra second your spartan takes when doing it (flipping off the safety, opening up a screen on the side, etc).This is plainly prevalent in power weapons, but was really prevalent in Reach. By "spawn switching," you won't have to deal with it later, thus more time you have on your side.
Fourth, learn to sidestep/ straf. find your own way of doing it, it just takes practice and time. Try to switch it up, and when having "hallway duals/ corner duals," swtich up the times when you expose yourself. So instead of crouch for 1 second, pop up and fire, repeat. Do: crouch for 1 second, fire, crouch for 2, fire twice, crouch for 1 second, jump and go after the person.
Fifth, learn the maps. Go into a custom game by your self or with a friend, and just jump around the map, looking for ways to quickly traverse it or find angle to see people coming. Look at some gameplay footage and see if anyone has found any jumps on the maps and practice yourself. Halo in past games and in a majority of Halo 4 is knowing the game and maps.
Sixth, find an "octogon" map. The idea is that you and your buddy can have some rapid pace duals in a short amount of time. You begin to learn how to line up shots better. This is better for non- auto matic weapons, and is one good way to learn to use the magnum effectively.
These are just my personal views, so take them with a grain of salt. But you should be able to get some key ideas from what i have said.