FREE UP SPACE ON MY MACBOOK AIR

You have a small drive by today's standards. There's only one way to open up space on any drive: delete users and or user data. You can copy the data to a cloud account of some sort, a good quality external drive (two for security) or put in on another computer which is a form of using another drive.

There are ways to optimize space, but if 90% of your drive is used, optimization is likely to have little effect. Unless the optimization is to move data.

Once user data copied to your cloud account or external drive and verified, you can then delete safely from the drive in question. Mac's are great machines but not magical. you have to take positive steps to actually free up disk space. And you need to leave approx. 20% of the disk always unused. The OS needs space to do its job. And on a 120GB drive, usable data space will be very small.

"Other" is controlled by the OS and you should not touch it. "Other users" (unfamiliar with it) may be left over user data if previous users were removed and you or someone selected to leave Home folder and data untouched. Are you the original owner? This is just a guess.

What disk size do you have? Depending on that you may have trivial concerns on disk space.

You can't manually clear "Other", that's controlled by the OS.

Are you asking how to remove a user and the associated data?