You must have a 3GS then? The 4 and 4S used micro SIM and then the 5 and up all use nano.
You go to any store or retailer like Future Shop/Best Buy/Mobile Shop/Wow Mobile etc. and ask them for a micro SIM. You can also order it online. They're $10 + tax on postpaid.
Once you have the new SIM, log onto your computer at home and go to Self Serve. Click on the Mobile Phone tab. Go to the bottom left where it says change phone and follow the instructions for activating a new SIM.