Do you have to get a separate plan if you buy the iphone?

I have been wanting an iphone for awhile now, and I was thinking about saving up my money to buy one. I just got my blackjack a few months ago though. my dad, my mom, my sister, and I are all on the same plan. can I just replace my blackjack with an iphone if i buy it? or do i have to get a whole new plan?