Whenever a subscription form is created, it will have only the default fields of First & Last name and Email ID. If you wish to collect any additional information from your contacts, you can add custom fields.
Follow these steps to add the custom fields :
Login > Marketing Tools > Overview > Subscription > Subscription Forms > Click on the "Manage" button next to the particular form > Add Custom Fields
A new page will load, you can click on the field of your choice and it will appear in the Form Designer area. In the Form Designer area, click on the "Manage" button, give the label name and save.