Yeah, rkaufmann87 does have a point, if all you see is Startup Disk, restart, shutdown and that’s it then something is definitely sketchy, eg the person/company you bought it from didn’t fully prep it for sale/for a new user… ensure an ethernet cable is plugged into your imac from your modem/router and that you have working internet service. Restart your mac, holding down the Apple + R keys together on the keyboard… you should see a language selection screen, pick one and continue…you should have a bunch of options.. to reset the admin/login password, move your cursor ( the arrow) to the top center of your screen and click. You should see “mac os x tools”. From there, go to “terminal” and click on it to get it going… you should see a white screen with black letters. Type in * resetpassword* but without the *’s , all one word, press enter/return on your keyboard…. That should get you into the main password reset application… ( that’s if you have a recovery Partition on your iMac) … if you don’t…then restart your mac, and hold down Option key+ Apple key+R Key all together on startup. …let go of ‘em when you see a spinning globe 🌎 and a progress bar on your screen… wait a while until the progress bar is all done.. then you should see a language selection screen, like in the first part of my post, then do the same thing to get to the admin/login password… also you may want to reinstall the mac os… but Finally, set your iMac’s build in hard drive as the startup disk.. before quitting the whole deal and restarting…
hope this helps
john b