F I buy a new sim card for a different company do I have to pay for a plan?

I have an unlocked, no contract iPhone 5s currently with a tmobile. If I buy a sim card from amazon can I put it in my phone and use their network automatically or do I have to pay for a plan?

In all phone companies you need to pay for the data plans. If data was free wouldn't everyone be using the free data now?

You will have to pay for a plan

Without paying you won't get any service, common sense here… Why would you think you would get free service, LOL.

You'd have to pay for a plan of course.

If you have an unlocked phone, you can use any SIM card with it. It will be activated automatically.