Here’s how to make a bootable installer of El Capitan on a USB Thumbdrive. It is the quickest way but it uses the terminal so it is for advanced users. If you want a way that is slightly longer but easier for beginners download and run Diskmaker which does the steps below for you automatically!
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
You will get a response like this:
Erasing Disk: 0%... 10%... 20%... 30%...100%...
Copying installer files to disk...
And after half an hour or so the USB drive will be a bootable OS X El Capitan Installer that you can use to install or upgrade OS X!
For more info check out the Apple Support article here:
If you have trouble with DiskMaker X here is a way to do it using the built-in Disk Utility in OS X.