Where do I go to buy more space on my phone do I only pay that once ? Or every month?

What do you mean by "buy more space"...? The storage in your phone is fixed. The capacity is what it is, it can't be increased, so if you bought a 32GB phone, for example, 32GB's your lot. That's why I'd always recommend budgeting for more storage than you think you're going to need. Same with computer RAM and hard drive space.

Or are you meaning iCloud storage...? iCloud storage has nothing to do with your phone's physical storage. For information on iCloud plans, please click the link above your question.

You don't, the storage space on an iPhone is a fixed size and can't be increased.

You can purchase more iCloud storage space but it won't increase space on your phone. If that's what you want them click on the link above your post.