Step 1:
Go to: Go > Utilities > Disk Utility
Double click on the Disk Utility icon to run the app.
When it opens, highlight your hard drive in the window on the left. it will actually be listed "twice", as the physical drive will show first, then the OS X partition will be just below the physical drive.
In the "main" window, there will be several tabs at the top you can choose. You want "First Aid."
Down towards the bottom of the window are a couple of buttons, one that says "Verify Disk Permissions" and one that says "Repair Disk Permissions."
You can run the verify function first and look at the results to see what it finds. It won't do anything at this point other than identify problems. Once you've run the verify, you can run repair and it will attempt to repair any broken permissions, reporting the results when it completes.
Once you've done this, if it reports that everything is fixed, reboot and try some things out that were broken before.
Report back after this.
MacDann