iphone activation and cheap plans?

I recently got an iphone 4 from my cousin, he recieved an iphone 5 so he took out his sim card and gave me his old one. I currently have a ghetto 20$ a month unlimited texting plan with verizon on a non- smart phone, without a sim card. How do i get my iphone 4 activated with either verizon or AT&T, i'm assuming i'd have to get a new number since i don't have a sim card.. and what are the cheapest plans verizon and/or AT&T offer?

Please Help :c