Ok, when you get Unity it comes with Angry Bots, a demo they give you. Create a new project, your new project will have everything you need to create the fps. Open the assets, Character Controllers, Firstperson Controller, then drag that into the mini screen to the left of the assets or drag onto the gameplay screen. Go to the top of the screen and click terrain.
Scale the size of the terrain to 1000000000 for all x,y,z. Then move the first person controller on top of the terrain. (the pill looking figure) When you click play the character will fall onto the terrain and you already can move around. You can can the speed of the movement by clicking on First person Controller then search around its properties and you'll find it. JediAction and beachbum, if you want to make halo games i'll send you guys a link to use as an example, or you can modify it.