If your computer won't support Sierra/High Sierra, you can try to down load El Capitan using the link below.
To check the model number hold down the option/alt key, go to the Apple menu and select System Information.
macOS 10.12/10.13 (Sierra/High Sierra) requires one of the following Macs:
iMac (Late 2009 or later) Model 10.1 and higher
How to get macOS High Sierra
Sierra download
El Capitan
Check to make sure your applications are compatible.
Application Compatibility
El Capitan 10.11 Compatibility information
macOS Sierra 10.12: Compatible Apps
Also check to make sure there is a compatible driver for your printer.
Do a backup before installing, preferable 2 backups on 2 different drives.