How to set up a used iPhone with my phone number?

I'm on Verizon and have a data plan for a Droid. I recently have purchased a used iPhone 4 from a friend of mine, and would like to use it as my phone. Can someone please explain to me how I can use the iPhone with my current phone number and potentially same data plan?

If you take it in to Verizon, they should do it for free. It might cost an extra $15 a month for having an iphone

Choose Settings, Phone. (Your number appears at the top of the screen. Choose Phone, Contacts. Your number appears at the top of the screen. Connect iPhone to your computer's USB port. When your iPhone appears in iTunes under Devices, click it and then click the Summary tab. So that you can set up a used iPhone with your phone number.