I want to listen to sirius radio but I need to use the iPhone app. How is this done? Not good with this stuff.

I want to Kristen to sirius radio on my ipad 3 w/retina. I guess I need the iPhone app to do this. Question 1. Why? Question 2. How do I do this? Question 3. I have an account with sirius already for my pc. But it won't go in the garage-not portable. The ipad would work great. Is it possible to work this out without having to pay for something else?