welll first of all sasono this is for you. you want to build a robot that follows a black line on a white floor this is too advanced for you now for your first bot you should the IR rangers for left and right obsticle avvoidence and put some bumber switches on the front and back if your using 4 servo motore you might want your robot to have tank steering so buy some relays, i really strongly do think loine following robot is too advanced for you i havent buuilt a robot but i am soon bacause the lack of money but thats what i think you should do