it's been recommended here to have ~50GB free space to upgrade to monterey. if you've already deleted what you can but still don't have enough space, you can purchase an external HDD or SSD of sufficient capacity and move your larger files (videos, pictures, documents, etc...) to it. once they are on the EHD, you can simply erase the same files from your internal drive. and, if it's just going to be used for storage, you can use an HDD and save a few bucks over the cost of an SSD. FWIW, i personally keep very little on my internal drive.
also, you may want to read these two articles:
Optimize storage space on your Mac
How to free up storage space on your Mac
and some food for thought, some people have had issues trying to upgrade to monterey from any macOS prior to big sur. if it was me, i would try upgrading step by step from your current macOS (mojave?) to catalina first and then big sur before trying to get to monterey.
the downloads for catalina and big sur can be found here:
How to get old versions of macOS - Apple Support
you may also want to read these two pages first:
How to upgrade to Big Sur - User Tips
How to upgrade to Monterey - User Tips