On a technical level, you need to find where the lines are in the image and then find the center point between them. If the center of the image is to the left of the center of the lines, steer right, if it's to the right, steer left. The critical points are the center of the camera image (assuming it looks along the vehicle axis) and the center point of the two lines.
You need to do some image analysis to find the center of the lines, the image center is just half the horizontal camera resolution. As the image is mapped to video memory space, it may be possible to scan addresses corresponding to a horizontal line in the picture to find the two lighter points or you could do it by sampling brightness at a set of points within the image to find them. It would be wise to scan several horizontal lines, the danger of scanning a single one is it can only find gradual bends in the path, if for example there was an abrupt turn, one of the lines could fall outside the image area and the center calculation would fail.
Brian.