Q: next after os x Lion?

The answer depends on which Mac you have, and how old it is. Can you tell us?

If you haven't been downloading all of the released OS's, you will have little choice as to what you can upgrade to regardless of what Mac you have.

Of the OS's after Lion and excluding El Capitan, Mountain Lion is the only one that can be installed--and you have to purchase it.

Deciding which of those two would depend on your hardware as Csound1 has asked.

Then you can (and should) upgrade to El Capitan (which is free from the Mac App Store)

Check to make sure your applications are compatible.

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

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.