There is no "installer" password per se.
When you went through the initial setup you created a password, this is your login password.
Unless you did something other than follow the screen prompts, that password is your Admin user password. When a password panel pops up, that's the one you enter.
If your "new" Mac didn't start up to a setup screen, you bought a used Mac that was improperly setup for resale. If that's the case, take it back to the seller and have it properly set up for resale: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support