I have a 5s, how can i pay for icloud storage if i cant get the icloud app, and why cant i get the app?

Which app can't you get ? If you mean the iCloud Drive app then that is available on the 5s, I can enable it and get it to show (Settings > iCloud Drive) on my 5s (iOS 9.3.5).

In terms of paying for iCloud storage, it's paid for by whatever payment method you have on the account, which you can view and/or change via Settings > iCloud > id > Payment

If you are using IOS 11, the iCloud Drive app has been replaced by the Files app.

Files app - Using