Halo 3 - why do I suck so bad?

Proably because I am used to WASD and a mouse and have no reactions ,
however...

I started it on Heroic the other night and got to the bit where you
have to save the downed pilot. I seemed to be constantly out of ammo
and using blasters and keep being sniped from the other side of the
map !

I restarted and managed to get better weapons and die less enroute,
but I'm still struggling in the same area. When I think I've got
further some big bugger comes and hits me with what looks like a
street light!

Whats the best way to play this and conserve ammo /die less?

Why is the stoopid alien buddy so crap at doing anything?

Thanks

Blonks
.