To install OS X EI Capitan

You must be using the administrator account in order to upgrade the Mac to El Capitan.

That requires the admin password.

First make sure your Mac meets El Capitan requirements > Upgrade to OS X El Capitan

If it does, you may be able to reset your admin password using your Apple ID > OS X: Apple ID can be used to reset your user account password - Apple Support

You must have an Admin account that you would have created. This is indicated in Users & Groups preferences.

Note that the second item is checked (and grayed out because it cannot be changed in the preference panel.) If you do not have a password for your primary login account then you cannot install software nor in some cases remove software. You can set up a password for the primary account by opening Security & Privacy preferences and clicking on the Change Password button that you see below.