How to Run First Aid with Disk Utility

Open Disk Utilities, which can be found in the Applications folder under Utilities

Once opened, a window will appear like in the picture.

Select the name of your Mac OS X Startup disk in the left column.

Click on the First Aid button at the top of the window.

A dialogue box will appear asking if you would like to run First Aid.

Click "Run".

This procedure will take a few minutes to complete. The machine may freeze up, or lock up during this procedure.

Once completed, click done.

Pro Tip: To see any errors that First Aid found, click on the arrow next to 'Show Details'.