If you want to upgrade your mac to a newer OS and you need to make more free space just move items you do not need on a daily basis to an external hard drive. Movies, Images and music usually take up the most room. After confirming that they have successfully copied over to your external drive you can delete them from your mac.
Which mac do you have, model and year, click on the top left and select About This Mac.
The year and model of your mac will govern which OS you can upgrade to.
If you have already bought a new mac then you can use Migration Assistant to migrate your data to the new mac.
Only migrate your personal data, do not migrate any apps as they may not work on the new mac and its newer OS.
New macs do not run 32bit apps, your older mac running macOS High Sierra is more than likely running 32bit apps.
Do not migrate any system or library data from the old mac to the new.