What is the error message?
Check that your computer is compatible with Mountain Lion/Mavericks/Yosemite/El Capitan.
To check the model number hold down the option/alt key, go to the Apple menu and select System Information.
MacBook (Late 2008 Aluminum, or Early 2009 or newer) model number 5,1 or higher El Capitan requires early 2015
MacBook Pro (Mid/Late 2007 or newer) model number 3,1 or higher
Your Mac needs:
OS X v10.6.8 or OS X Lion already installed
2 GB or more of memory (More is better - 4 GB minimum seems to be the consensus)
8 GB or more of available space
Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.
Application Compatibility
Applications Compatibility (2)
El Capitan 10.11 Compatibility information
Do a backup before installing, preferable 2 backups on 2 different drives.
If you can/do upgrade, I recommend you make a copy of the installer and move it out of your Ap