I don't know where to get Apple certification.
Just to be clear, "certificate" in the developer parlance is a cryptographic entity that allows you to prove who you are mathematically. To get a certificate from Apple to sign your App Inventor projects, you must pay Apple's developer license fee and set up your Apple developer account. Once you have done that, follow the instructions here to create the provisioning profile you'll need for App Inventor to sign apps on your behalf for the App Store.