Well, I can't just help you design the robot. You'll have to do it yourself, and it'll be quite an easy task using the tutorials on this site. Here is the step-by-step tutorial on how to create your first bot:http://societyofrobots.com/robot_tutorial.shtml
After you've done that, you might want to move onto a new (since you're going to build a line-following robot) type of sensor.
It isn't something I could just tell you, it takes lot of planning, you'll run into problems, bugged code etc. You'll have to learn it step-by-step!
Hope this helped you a bit (yes, I know this isn't the answer you expected to get