Before uploading code, you will need to create a compute capsule to contain it.
After logging in, navigate to https://codeocean.com/dashboard, where you will find the New Compute Capsule button. Click it and choose the appropriate language (in which most or all of your code is written) from the drop-down menu.
(You can add more languages later if you need to. See Setting up additional languages on Code Ocean to see how.)