Everyone that plays Halo 3 starts with the same Spartan. It's Master Chief, there is no other Spartan you play as. You'll end up getting access to better, more powerful weapons as you play. If you got a really bad gun, you can always go up to a member of your squad and swap weapons with him if he has a better one. Try to stick with the Battle Rifle as it is the best all around weapon to use. Pistol or SMG as your secondary weapon.. I prefer the Bruteshot though.
Also, adjust your controller settings if the aim is off. You can change the sensitivity to your liking. Just keep playing around with it, you'll be a pro in no time.
Cheers!