If you joined in-store, you'll need to activate your online account. Your welcome email contains a link to do this. Or use the Sign In / Register link on the top menu of the website and click the Activate button. You'll need to use your original email that you registered with in-store to set up a password.
If you registered on the website, your online account was activated during registration. You can use the Sign In / Register link on the top menu to log in to your account as a returning customer. Use the email and password set up during registration.