Every time you render a frame, you have to specify your projection matrix and place your camera position, and the direction you are looking in. There is nothing special or different about a top down, side scrolling camera. Just put your camera where you need it, and then tell it what direction to point in.

Create a camera class, and use it to keep track of the camera's position, lookat vector, Field of View, and anything else you might want, like velocity.

http://nehe.gamedev.net/article/camera_class_tutorial/18010/

Every time you render a frame, you have to specify your projection matrix and place your camera position, and the direction you are looking in. There is nothing special or different about a top down, side scrolling camera. Just put your camera where you need it, and then tell it what direction to point in.