It will vary depending on a number of things. For the most part it will start with 'com.app' and then will have your app name compressed after.
For example, if you want to call your app "Cool Hats" your app ID is going to be:
com.app.coolhats
If you have your own business URL, company or brand you can use that.
Say you have a gardening company called 'Greens' and an app called 'Greens Gardening' you can opt for an app ID as follows:
com.greens.greensgardening