Not enough storage for El Capitan; no way to free up space
Boot to Recovery Mode (hold cmdR while booting). In the menubar open Utilities -> Terminal.

Now change your working directory and go to /Volumes:

cd /Volumes


List all volumes:

ls -l


Move to your main volume

cd name_of_main_volume


Now you can move forward to a directory to remove files and folders with cd folder_name.

Appropriate paths to remove files or folders without touching your own data are:

/Volumes/name_of_main_volume/cores
/Volumes/name_of_main_volume/var/vm


Then remove files with rm file_name or rm *.* after making sure you are in the proper directory with pwd and checking all files with ls -l.

The folder cores may be empty, vm usually contains a sleep image and/or swap files which are all disposable.

If this doesn't suffice go to your applications or your user folder and remove files/folders.

To remove a folder use rm -R folder_name. In this case application files (e.g. Maps.app) are also folders!