Running Cubase 8 on High Sierra is not officially supported. It will probably still work, but there could be issues caused by the changes made by Apple. Steinberg wont be able to release updates for Cubase 8 to fix them, and they might be unable to help you if you need technical support with this version and OS combination.
You can upgrade from any version of Cubase LE, or Cubase AI, or Cubase Elements, to the latest version:
If you have Cubase LE or Cubase AI, log into your MySteinberg to check if there are any upgrade offers for you. But I would recommend upgrading to Cubase Elements 9.5 instead if you can afford it. It gives you a lot more freedom and updating to any future version of Elements is cheap ($19), even if you skip versions.
Compared to Cubase 8, Cubase 9.5 offers noticeably improved graphics performance on Mac along with several workflow enhancements.