Switching number from Verizon to Straighttalk AT&T. How to keep my old number?

Right now, I have a Verizon noncontract phone. I am buying an iPhone 4 from a friend which is on the Straighttalk AT&T micro sim plan. I am moving to the Straighttalk AT&T plan and need to know how to get my number changed over and where to get a new micro SIM card from. Please help me!