How do I remove all the files on my computer?

this is a description of how you get it back into the state it was when you purchased it

How to erase an Intel-based Mac - Apple Support

Get an external HD that's big enough to store your larger files to and use them from there and leave lots for additional files (I'd start with a minimum of 1 TB). Format the drive as shown in this screenshot depending on what type of drive it is:

Then drag the files and folders that you want to move to the EHD from the Documents folder to the external HD. Before deleting them from the boot drive make sure they are accessible and working on the EHD.

Excellent candidates for an external HD are these:

Envoy Pro EX with USB-C or USB 3 - best from a speed vs price standpoint

OWC Envoy Pro EX with Thunderbolt 3 - fastest if you have a Thunderbolt 3 port but more expensive

OWC Mercury Elite Pro 7200 RPM Storage - most economical but slower than the others. Still good for casual storage and use. Also excellent for Time Machine use.  

first, ensure that you have finder preferences set to show external drives on your desktop. then open a finder window for your internal drive. then double click on the icon for the external drive and a window will open for that drive. once the external drive's window is open, you can drag and drop your files from your internal drive's finder window to the external drive's window. you can do it one by one, or "select all" and move them all at once. depending on wether the internal and external drives are HDDs or SSDs the transfer may take a while. once you've verified that all your data was successfully moved to the external drive, you can go about removing those same files from your internal drive without affecting the items that are now also on the external drive.

Are you selling your Mac, do you want to totally factory reset it, or do you just want to free up storage space?

Cheers,

Jack