iMac 8.1

Here are detailed instructions on upgrading to El Capitan which if you provided correct information for your computer model is going to be the highest macOS it will run.

Upgrade to OS X El Capitan - https://support.apple.com/HT206886

Check if your apps will still work with El Capitan - https://roaringapps.com/apps

Back up your data - -> Back up your Mac with Time Machine - https://support.apple.com/HT201250

In particular, get information about your computer and make sure you have enough RAM to run El Capitan. Apple says 2 minimum, but in reality it will barely run with 2. Your computer can likely take up to 6 GB total and I recommend you find out your exact model and abilities here: https://everymac.com/ultimate-mac-lookup/?search_keywords=iMac8,1

To get more information about your computer, go to the Apple in the upper left corner of any window, then "About This Mac", then "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).

As for Facetime, I never use it on a computer. It is hard finding system requirements for things such as Facetime because Apple assumes everybody is running all the newest hardware and software so they don't bother posting that kind of information. While El Capitan isn't the newest system, it isn't't all that old either and should support Facetime for your needs. I use Facetime with older software versions on my mobile devices.