This step is SUPER easy!
Basically, every app needs a unique bundle identifier so that your app can be found out of the millions of apps in the App Store. The App ID is made up of 10 characters given to you by Apple as a prefix + an additional string of your choosing.
Ok, at the main page on the Apple Developer Website, click on the Certificates, IDs & Profiles tab. Then click on the App IDs tab.
Above you'll see I already have a few App IDs. To create your first ID, click on the + button in the top right
This is where you'll add your product or app name. After you enter your App ID Description name make sure that you choose Explict App ID also remember to use a reverse domain name string for this (It's recommended)
You can use the default App Services (they're adjustable later) and Continue & Register. Once you're done with that, you can now proceed to creating development & distribution provisioning profiles.