You can set up your signup form so that new subscribers are automatically placed into a list of your choosing. First, click on Sign Up Tools > Link to Your Signup Form. Then, click on List Settings on the left hand side:
From here, you can choose 1) whether to Show/Hide the list name on the form itself and/or 2) which list to automatically assign new subscribers to if you don’t want to add list options to your form. You can auto assign to more than one list if you like:
Save changes, and you’re done!
If you’re embedding HTML, you have the same option: