Make Sure You Have Room for the Update
You should have 8GB of free space on your Mac to install the OS X El Capitan update.
Check to see how much storage you have in Finder by clicking Go -> Computer -> two finger click on Macintosh Hard Drive -> Get Info.
While Apple only suggests 8GB, it is a good idea to free up more space than that. Aim for at least 15GB free to handle moving files around. Here's how to cleanup your Mac hard drive.