Halo 2/Cairo Station

Easy
The first thing you should do for this level is the obvious. Go down the stairs, and you will find various weapons. They are the: Battle Rifle, SMG (dual-wieldable), and the M6C Magnum. When you walk over or close to these weapons, you will pick them up. Once you've chosen your weapons, walk over to your marines, and they will go through the next door to your first fight.

Walk down the stairs directly in front of you, down to where a Sargent is manning a turret. The door you see across the room is where your enemies will come from.