To purchase additional storage, go to "Settings" on your phone, tap your name, then click "iCloud." Here you should see exactly how much storage you've used and how much of each data type is contained in your iCloud storage. Click "Manage Storage," then "Upgrade" to upgrade to iCloud+.
If you're skeptical about the subscription prices, remember that iCloud+ comes with premium features aside from more storage space. These include "Custom Email Domain," "Private Replay (Beta)," and "HomeKit Secure Video Recording," among others. If you don't want to upgrade, back up your data on your PC.