Basically, open Disk Utility. On the list in the left will be at least one disk. Click the one second from the top (it should have the same name as your hard disk and will be indented).
Then click Repair Disk Permissions in the bottom left of the main part of the Disk Utility window. You will NOT be able to click Repair Disk. This is fine. Hopefully, that should repair your disk permissions and fix your problem.