I'm looking at a phone and it says data service is required. Does this mean that I have to pay for the 4G or the 3G or whatever? Please let me know. Thanks!

It means you have to have a plan that will allow for "data" to be transferred, while not always the 3 or 4 G ( those are plan names for amount of transfer speed) but yes your plan has to include data

Yes you will need a 3g or 4g service plan along with your voice plan.