New old iMac

I'm new here, so hi, and please be gentle. I'm not a native Mac user, I have had to learn for work I do at my church.
I just purchased a used iMac and of course it wants my password, and of course I don't know it. Now what do I do? Also how can I upgrade the OS? it has OS X.9.5 I think.
Thanks for the help, I have lots of questions, so I hope you don't get tired of them.

Welcome to our forums.

We need more information. Which iMac did you buy? Year and model? If the operating system on it is macOS 10.9.5, that's "Mavericks".

Is it possible to contact the seller and get the password? We can show you how to reset the password as long as it's not a password that was set in firmware.

Thanks for answering. It is an iMac 20 inch with 2.66 GHz Intel Duo Core processor. Model A1224 iMac8,1
I don't think I can get the old password. I purchased it at a rummage sale where I live, and even though the old owner still lives here, I spoke to her once and she hasn't used the machine for years. She got a Mac Book and never came back to the iMac. But I will ask her. There are lots of personal files on it and I want to wipe it out and start over. I also have a Mac mini that has files I would like to move over. I'm thinking of getting a pro to give me a quote for all the work.

Start up in Recovery mode, https://support.apple.com/en-us/HT201314. From there you can erase the drive with Disk Utility, then install the OS. After you install the OS, you can transfer all your data during setup using Migration Assistant.

What Mac OS X version is the mini running? The iMac will run up to OS X El Capitan (10.11). So, if you don’t already have that in your purchased section of the Mac App Store, see here to download it, https://support.apple.com/en-us/HT206886.

Wow, thanks so much. You make it sound so easy, LOL. I have done this to lots of PC's so I can try on this Mac. The mini is running El Capitan, 10.11.6. From reading other posts I understand that it is best to use a cable, FireWire?, for migration, YES?

Ethernet will also work. But, if you have a FireWire cable, that is fine too. You may have to use Target Disk Mode, if your using a FireWire cable.

https://www.makeuseof.com/tag/target-disk-mode-guide/