I am completely new to halo 3, so if anyone can give me any tips about just about anything in halo 3 (preferably slayer), that would be great.

Well a few tips I can give you is to constantly check your motion sensor, practice using the BR (Really powerful weapon once you've mastered how to use it). Also never hide, keep moving so you can get more kills and most importantly never use the Assault Rifle to shoot someone from across the map. I hope this helps a little. :)

Just keep playing, practice practice practice and you will learn :)

I agree with titan in keep moving, still targets are easy for snipers and spartan lasers especially

(BR I believe stands for battle rifle just in case you were wondering)