The Repair Disk Permissions function is part of Apple’s Disk Utility (in /Applications/Utilities). After launching Disk Utility, select the desired disk—generally your startup disk—in the list to the left, then click the First Aid tab. At the bottom of the First Aid panel, click the Repair Disk Permissions button.
To find Disk Utility, go to Applications. Inside the Applications folder you will find the Utilities folder. Open that and double-click on Disk Utility (see below).
Once it is open, select the name of your hard drive. Originally it was named Macintosh HD. It may be named anything at this point, but you will find it in the same location that you see Macintosh HD in the image below. Click on Repair Disk Permissions.