Tried playing Halo 3

Well, I tried playing Halo 3 last night. And man did I suck! Im not sure if it was because it was my first time playing, or if it isnt just my cup of tea! One of the things I noticed most, was the amount of ammo it took to kill one person. I would sometimes put 2 clips in another player, but nothing would happen! Maybe I just cant aim, but I really doubt it.

Any feedback on this?