How do you check my disk ?

This normally means your hard drive is failing, back it up as SOON as possible then make a trip to your local apple store.

You need to boot from a setup partition if you have one and run disk check. Restart the computer, then as soon as you hear the opening chord hold COMMAND+R until the Apple log appears. This should boot the setup partition. Run Disk Check, click on your primary disk and then Repair Disk.