How do I clean my disk correctly?

I have a Gears Of War 2 game I wanna play but every time I put it in it told me I needed to clean the disk. So I took out my glasses cleaner liquid and my cloth which made it work until I played with people on live. When I wasn't even shooting a locust or not even taking one step it said it was unreadable. Help me! I really want to play the game but can't because it is saying I can't.