Please i use iphone 6 with ios 11.4(15F79) how do I delete old photos and videos from my iCloud since its full and i am not set to purchase more storage for now?

See if this helps

Manage your iCloud storage - Apple Support

iCloud works more to sync content and keep copies of content. It’s not a separate backup, at least not in my experience. If you want to keep what’s out there you’ll need to move it elsewhere, then delete it.

The cloud is also what you have stored on your device. So delete stuff off your device and it should delete it off the cloud.