how can i buy space for my i phone

You can't, it is a fixed size that can't be changed.

If you need more room, you will need to delete something.

Here are some helpfull articles that can guide you on how to free up storage space on your phone.

How to check the storage on your iPhone, iPad, and iPod touch

https://support.apple.com/en-us/HT201656

Manage your photo and video storage

https://support.apple.com/en-us/HT205703

Clear Other storage on your iPhone or iPad

https://support.apple.com/en-us/HT211686