unfortunately there is no simple solution :(
1. Open your Xbox and CD drive. Clean all optical elements on it.
2. If it did not help find 2 small potentiometers and try to increase voltage on the laser.
3. Replace your cd drive - but you need to reflash a new one with the old cd key.
If you don't understand how it works it will be much better to go to any repair shop - I don't know how much it costs in the US but in London its about 20 quid (just the labour)
Peter