Naturally you must purchase a dataphone for your smartphone which should not be a problem since all smartphones sold by Verizon, AT&T, T-Mobile, etc. requires such a plan.
Some smartphones have hotspot capabilities while others do not. Using the wi-fi hotspot feature generally also requires an additional monthly fee (perhaps $10 - $15) which you need to check with your carrier. I believe it also increases your monthly data plan limit, but again you must check with your carrier.
Anyway, using wi-fi hotspot means you will be using the phone's data plan. Going over the data plan limit usually involves additional fees or the transfer get reduced (like from 4G down to 2G). I think wifi hotspot can allow up to 5 different devices to...