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

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

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).