Advice on getting set up to receive credit card payments

I’m going to have a booth at the Texas Homeschool Expo in May. This is my first time to participate in an event like this. I assume I should get some sort of device for accepting credit card payments. I’ve heard you can get one for free and only have to pay the 3% fee on each purchase. Does anyone have any idea how to get this or have a suggestion as to which devices are more user-friendly, etc? Thanks.