To add a membership plan against a new customer's account you would need to go to the "customers" page > "members' tab " > "add member":
Select on the "customer" field:
And this would then allow you to select on the "Add new customer" button:
Fill in the information needed for the new account, and select the plan:
You should then be able to fill in the Card Information to charge against the customer's subscription.
Optional: you can also use or apply a discount to the membership purchased by the customer. This would allow you to apply the promo code that you have set up for memberships or indicate a discounted price.
Here is more information on how to set up promo codes for your membership plans: