Are you really new to LoadRunner tool? or say performance testing in general? If Yes, please be advised to start performing R&D and learn basics of Performance Testing. This will help you to learn Load runner and performance testing as well.
Now as far your query is concerned, I would suggest you to first investigate the protocol on which your STB will work. As far as I am able to understand, this won't be HTTP alone. Verify the specifications of connectivity and channelization of communication between STB and the other source as it essential for its working.
Then try to search, how you will use Loadrunner to hook in between STB and source and act as a performance testing tool of choice.