before you begin with selling memberships, you should get familiar with security assist, especially user Level Authentication.
On the security assist support page, look in the archived documentation for a tutorial on user level authentication, it was written for Security Assist 1, but the concept is the same.
the different membership levels you sell will be essentially different user levels.
on the eCart support page, see the tutorials for adding registration to the eCart checkout pages.
the way this will work is:
The registers for basic access to the site. the userlevel should be set to only allow checkout.
Once the checkout process is complete, update the userlevel for the user to the membership level they have purchased. This can be done using the Data Assist update record behavior and the UserID session variable that is created.