Once you have your New customer criteria in place, you'll be able to create memberships which only apply to new customers.
To set this criteria, head to Memberships>>Memberships.
From here you can either add a new membership, or edit an existing one.
From the create/edit form, you will see the option to set the membership for new customers only.
Only customers tagged as 'New' will be able to view and purchase memberships with this setting in place.