I need help with my xbox 360 game..?

Hey. i have grand theft auto for my xbox 360, it plays and i get into the game, but when i start driving around, it says i ned to clean the disk with a soft towl or something, says it about 5 mins into the game. i dont get it, ive tried cleaning it so many ways (toothpaste didnt work) and i dont have ingrediants for some others -.- , PLEASE HELP