It's very easy. You have 2 options:

1. In your user account, by clicking on  Open my store  and following the steps in the wizard.

2. In the "  Create your store" section , click the button below that says "Register my store!" (if you are logged in; if not, enter your email address there).

Once your store is registered, you can proceed to add designs, slogans, t-shirts, and anything else you deem  appropriate .