Fields such as the one pictured below (e.g. shirt size, primary position, doctor, doctor's phone number, etc.) are known as custom fields.

You can edit custom fields in the admin panel by navigating to Setup->Registration->Registration Setup Wizard. You can add fields to either the parent page step or the player step. Choose which tab you would like to add questions to and fill out each of the fields that appear in the screenshot below:

Don't forget to click add field when finished.