No, you don't need to have an account with any carrier before you can start to use your Shiptheory account. In some cases, you will spend some time first setting up your carrier accounts and the integration with your Shiptheory account which might take you a few hours to a few days.
If you don't want to waste time and want to run some tests (given that your channel is already set up), you can use our Address-only Shipping Labels integration.
Need help with anything else, feel free to contact support.