Can you upgrade to Sierra?
To check the model number hold down the option/alt key, go to the Apple menu and select System Information.
macOS 10.12 (Sierra) requires one of the following Macs:
MacBook (Late 2009 or later) Model 6.1 and higher
MacBook Pro (Mid 2010 or later) Model 6.2 and higher
Or El Capitan
If it won't let you, then your only options are to upgrade to Sierra/High Sierra or find an older computer where you can download it using your Apple ID.
Check to make sure your applications are compatible.
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.