Step 1: You need to create a profile for yourself. "Manage -> Profiles" in top right navigation and click the Create New Profile Button.
Step 2: Add your Athlete Membership to your cart. "Manage -> Memberships" in the top right navigation and click the Purchase / Renew Membership Button, select your profile, and then the Athlete Membership.
Step 3: Pay for the membership. Under the Cart icon in the top right navigation.
Please watch the linked video for a step by step tutorial on how to purchase an Athlete Membership.