If you have WiFi on a cell phone do you have to buy a data plan?

When you buy a cell phone you also need a company to provide cell phone service to that phone, so yes, you will need a plan in order to receive cell phone service.

However, there are two kinds of plans. One that requires a contract and one that is called the "pay as you go" or "prepaid" plan. So you do not have to have a contract when you buy a cell phone.