I want to reset my Mac mini like new

That depends on the year model and why you want to start over.

If it is a 2012 or later and you want to sell, give away or just start over fresh. You will need to use the following instructions to erase and reinstall the macOS.

see > What to do before you sell, give away, or trade in your Mac - Apple Support

If it is older, then you will need to use the original Install disc set that came with that Mac Mini to erase and reinstall the OS X.

Keep in mond that when you erase your Mac's hard drive, any apps you have purchased and installed will be gone and some may have to be upgraded and repurchased to newly install them.