I'm pretty sure you must have set a password when creating your initial and only user, the fact that you think you might not have a password is that you likely have Automatic log in set for that single user, so you've never been asked for a password when starting your Mac.
Turning Automatic log in off will ask you for the password whenever you log in, probably what you are trying to achieve. The big problem here being that you don't seem to remember which password you might have chosen during the initial setup.
Any possibility it might be some password you prefer to use for any of your Macs/iPhones … best to try changing the password until you get the correct old one accepted, then make sure to enter a memorable new one and only turn Automatic log in off then.
Alternatively create a new user, also with Admin rights, move all your stuff over to that new user, then delete your old 'forgotten password' user, leaving you with a single user again with a known password for you to turn off Automatic log in.