Activate SIM as new line?

I bought a new phone at the Apple store, and they gave me a T-Mobile sim card with it, which I put in the phone.  How do I activate this SIM as a new line?  I can see options on the website to order a new SIM (which seems unnecessary) or to change the SIM for an existing line (which I don't want to do).  I even tried ordering a SIM with the hope that it would give me the option to change the SIM to the one I just got, but it doesn't work.  It seems crazy that there would be no way to do this.  Do I have to call support?