Does unlocking an iPhone still make you pay for data?

I'm planning on buying an iPhone off of craigslist, and it unlock after. If I unlock it, will I have to pay for data if I don't put in my sim card right away? I know how to unlock it, I just don't want to pay the data plan for a smartphone and not get caught.

I don't understand, by unlocking your phone your able to use other sims inside your phone. But if your on contract, it doesn't matter whether you put another sim in or not, you will simply be charged a certain amount every month and that will be taken from your account via direct debit.
By unlocking, you don't get free internet? You still have to pay! Just to another sim provider.

If you unlock it, all that will do is allow you to use it like any other phone, just on a different carrier then it was designed to be.So, if you unlock it you will still have to use the SIM card and pay for the data as normal.