Go to Settings
Tap your name
Tap iCloud
Tap Manage Account Storage or Manage Storage
Review what is using the most space
Delete old iCloud backups
Turn off backup for apps you do not need
Delete photos and videos from iCloud Photos
Remove photos and videos from Recently Deleted
Delete large files from iCloud Drive
Remove unnecessary documents from Files
Delete old messages and attachments in Messages
Turn off Messages in iCloud if you do not need it
Delete unused app data stored in iCloud
Clear Mail attachments if using iCloud Mail
Remove shared albums you no longer need
Upgrade your iCloud storage plan if needed