how to buy online with apple account

This document outlines what you can buy with your Apple ID [Apple Account] balance --> What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

As you can see, there's numerous things and it would be a long list to say exactly how to get each kind. A few examples:

Download apps and games on your iPhone or iPad - Apple Support

To learn how to buy music from the iTunes Store click here --> Buy music from the iTunes Store - Apple Support