After you've added a contact form on your site, make sure you change the layout so your customers submit only the relevant info. Add and remove the fields in the form, make some of them mandatory — that is up to you.
Customizing the layout of a contact form
1- Hover over the contact form and click on the "gear" to access its settings:
2- In the Layout tab press Add field and select the fields you'd like to show in the form — email, date, check box, etc:
3- To access the settings of the field, click on the "gear" next to it: