The more you use your phone, the more you fill your iCloud Storage with photos, backups, and other data. And since Apple currently offers a paltry 5 GB of iCloud Storage free with each account, it’s easy to fill that up.
You have two choices: reduce the amount of data you put into iCloud Storage or buy more space from Apple.
Buy More iCloud Storage
If you haven’t done so already, I recommend you purchase more space. Currently in the United States, $0.99 per month will upgrade your storage to 50 GB. If you need even more space, Apple offers two larger iCloud Storage upgrades.
If you purchased extra storage space before September 16, 2015, you may have a yearly plan that gives you less than 50 GB but more than the free 5 GB. These are no longer offered by Apple, so if you switch to a cheaper monthly plan, you will not be able to return to a yearly plan. For most people, switching will get you more storage for less money.
It’s easy to buy more iCloud storage (adding or changing plans). You can purchase iCloud Storage plans on your iOS device in the Settings app (see Settings > iCloud > Storage) or on your Mac from the iCloud pane in System Preferences. Apple’s instructions for purchasing more iCloud storage also explain how to do this on older iOS versions and Windows.
Reduce Your Data
If instead you decide to reduce the amount of data you put into iCloud Storage, Apple explains how to see what's eating your space and how to make more space. Videos and photos are likely culprits, but backups of discarded devices are worth looking for, too.
References: Apple Support Articles
iCloud storage plans and pricing
iCloud storage upgrades and downgrades
Manage your iCloud storage