How do I get more storage?

On your device itself? You buy a new device with more storage. Device storage on iPhones and iPads is not upgradeable. If you mean iCloud online storage, you can subscribe for a larger quota -> iCloud storage plans and pricing - Apple Support

Instructions are in -> Buy more iCloud storage - Apple Support

(btw, blue links in posts such as Elitegate’s post are links to documents, click on them to follow them for info).

It's literally in the document. Also on which device you do wanna buy more storage?

On iPhone:

On your iPhone, iPad or iPod touch

1. Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage.
2. Tap Buy More Storage or Change Storage Plan.
3. Choose a plan.
4. Tap Buy and follow the on-screen instructions.

Buy more iCloud storage

https://support.apple.com/en-gb/HT201318#ios