Ok. First of all, if you play CS you really need to pay attention.
1. Weapons
If you want a good weapon combo, make sure you either know what your doing and/or always have the human pistol.
If you do know some what of other suitable combinations of guns go ahead.
2. Movement
Unlike Counter-Strike, you are always as accurate as you are going to be. Crouching only helps conceal you, not give you better accuracy.
Jump a lot! Jumping does not screw with accuracy, actually, nothing in this game messes up your accuracy.
Sidestepping is important also. The old gaming technique of sidestep running is not true but sidestepping will help you survive.
3. Vehicle and Anti Vehicle
ROCKET LAUNCHER YOU FOOL!! Grenades are effective also, but Plasma grenades are the premium anti-vehicle. If they are in a warthog, you can just fire a lot provided they don't run you over. If a ghost is charging you, then get on a side of a hill (so you still can see them coming) once they come over the hill, duck and they will fly right over you (this also works on the flat ground, sometimes).
If you are in a vehicle being shot at, swerve around. If you know where they are, fire at them or charge them. Also, despite what you think, try death charging instead of waiting for them to run out of grenades, or them to reload.
4. Snipers/Sniping
The only disadvantage of the sniper in Halo, is that the
sniper rifle bullets are mini-rockets, and that means they leave a smoke trail. That is handy to know when sniping or when being sniped.
If you know any military sniper, they tend to move after a kill. I suggest the same tactic for Halo. If you shoot at a person and miss, make sure you either shoot back quickly or hide.
5. Base/Flag Defense
If you've been assigned to base/flag defense, the 2 guns you want are the
assault rifle and the shotgun. The shot gun is the ultimate in close
range combat, and the close quarter camper's weapon of choice. In Blood
Gulch, if you can get both the warthogs, block the base ground entrances
with the warthogs. On Sidewinder, you can sit on a cliff with a tank and
that is effective. Camping and staying low in the base until the flag is
touched is critical.
6. Using the tank correctly
The tank is a powerful asset to any good multiplayer game. It is often
used in correctly (which makes me angry). Some people think they are
invincible in the tank because...well...it's a tank. WRONG! The driver
needs to listen for rockets, grenade noise and even other vehicles (each
vehicle has a different noise). Artillery is very effective in Blood
Gulch and Sidewinder. The tank is a very good "assault the base and own the
other team while capturing their flag" vehicle. Remember to listen and fire when
you see te person NEAR your crosshairs, it's an explosion, it will have a lot of splash damage.