Are you talking about iCloud storage you need more of, or space on your mac system drive.
If you need more iCloud space then you have to pay for it.
If it is space on your mac you need, you have two choices. Switch on "optimise mac storage" so that full size originals are moved off your mac and stored only in iCloud, or move your library to an external drive. (See:
See
Move your Photos library to save space on your Mac – Apple Support (UK)
Or you can do what I do and have two libraries:
I run two libraries. I pay for 200GB of iCloud, and have a "main" library holding all my recent photos, that is synced with iCloud and resides on my system drive. All these photos are available on all my devices. Currently I have 1 1/2 years of photographs there, taking up less than half of my iCloud space. (This dual library strategy is equally valid if you don't use iCloud but just want a smaller local library stored on your mac)
I also have an archive library which holds everything older. It is on a separate drive (correctly formatted and prepared - see link below). I use powerphotos to periodically copy photos from my main library to my archive library.