How do I download something on my Apple device?

Here is the part of the iPhone guide that introduces the App Store:

Get apps in the App Store on iPhone - Apple Support

The rest of the iPhone guide introduces a whole lot more about what you can do with iPhone.

iPhone User Guide - Apple Support

Apple also has a support article on using the App Store:

Download apps on your iPhone or iPad - Apple Support

If some error such as an Apple Account disabled in the App Store is arising, or if you have other questions, please provide a few more details about that.

Read this document to learn how to download apps and games on your iPhone or iPad

Download apps on your iPhone or iPad - Apple Support

To learn how to buy music from the iTunes Store on your iPhone or iPad click here --> Buy music from the iTunes Store on your iPhone or iPad - Apple Support