You seemingly need more storage. How much more depends on how much you already have, and what is stored in it, and how much of that you need and how much you don’t need anymore:
… Manage your iCloud storage - Apple Support
The simplest storage in a computer is a bit. That bit represents either On and Off, or Yes or No, or True or False.
That simplest storage is then combined into eight-bit hunks, and that is known as a byte.
A single byte can represent a single common English letter, or a number from 0 to 255, or a signed number from -128 to +127.
As bits were combined into bytes, bytes of storage are combined into yet larger pools. That might be to store a larger number, or to store more letters, or to store letters from other languages, or emojis.
A kilobyte (kB) is a thousand bytes, a megabyte (MB) a million bytes, a gigabyte (GB) a billion bytes, terabyte is a trillion bytes, and a petabyte is a quadrillion bytes. (I’m also deliberately glossing over some details here, including the difference between decimal and binary storage measurements.)
A double-spaced typed page of English text requires about two kilobytes of storage.
Images can require anything from a few kilobytes or so to enormous volumes of storage (here’s a gigapixel-sized image of New York City—you can zoom and zoom that image, and then zoom some more), depending on how big or how detailed the image is, and how efficiently the image data can be compressed. An iPhone 15 image is about 15 megabytes, when using a fairly efficient image format. Other cameras and other formats and other images can be larger or smaller. A gigapixel-sized image is… much larger.
And as internal storage and iCloud hosted storage a common source of confusion:
… What's the difference between device storage and iCloud storage? - Apple Support
One of the things that uses some iCloud storage and that you will want are iPhone backups, too:
… How to back up your iPhone, iPad, and iPod touch - Apple Support
