Your Mac Mini can run High Sierra, OS 10.13.6, but that's it. If you want to upgrade, you'll need to get prepared. Here are the steps/tasks you need to perform:
1. First, are you making backups to an external device? If you are, what software/program do you use for that? Time Machine is OK, but SuperDuper! or Carbon Copy Cloner are better.
2. What third party applications do you use? And have you kept them up to date? If you do "upgrade" to High Sierra, you'll need to insure that all of them are compatible with High Sierra. That might require upgrading some/all of them. This site can help with that:
Crowd-sourced application compatibility for macOS, iOS and Windows.
roaringapps.com
3. Have you ever done any disk cleanup/maintenance/repairs? You actually can do quite a lot of disk cleanup on your own, and there are some excellent free/commercial programs to help with that. Onyx is an excellent, free program to help with that (
https://www.titanium-software.fr/en/onyx.html). Make sure you get the correct version for the OS you are using. You should also consider investing in a commercial program. TechTool Pro is excellent (
https://www.micromat.com/products/techtool-pro). IN fact, I depend upon both Onyx and TechTool Pro.
Those products can also give you an indication of the health of your internal drive.
Also, deleted EMails is another area. When you delete an EMail with your EMail client (Apple Mail, for example), the deleted EMial is still on your drive. You really need to permanently remove deleted EMails you no longer need. Each EMail program provides a way of doing that. I use Thunderbird as my EMail program, and it is easy for me to do it.