You should wait for your biometrics letter. If you have access to your ELIS account (they send this in a separate letter with your code), you can also pull up an online copy of your biometrics letter. If you never receive either of these letters, which has happened to me, you can contact USCIS Help hotline early in the morning, like 8 AM on the dot. They will transfer you to an agent who can give you an ELIS code. If you call anytime after 8:30 AM, you run the risk of the agents being too busy to accept any transfers.