You can use a camp cook stove, propane stove or even a little Ben Franklin stove if you would like but a good fire pit just can't be improved on. Depending on the size of the tipi, and the number of people who will need foot space, you usually plan on a fire pit of at least 2 feet in diameter by 1 foot deep. You can make it 18 inches deep and add rocks to a depth of 6 inches to really keep it warm. This is what usually happens to folks just learning. Always make sure that the fire is either going before frost sets in or don't use a fire inside during the winter, because convection currents from body heat will keep it warm enough to sit in with a jacket on, even in snowy cold, if you are only going to use it intermittently.
Fire pits work much better that way than a regular fire.
Our fire pit in the backyard is rock lined, with cantilevered rock shelves at various heights for cooking and warming, and is 1 foot deep. It heats the ground to a diameter of about 6 feet, quite nice and toasty for fall cookouts. If you get the coals 13-18 inches deep you can keep it hot from for most of the winter, using very little wood. We do caution you NOT to use river rocks as they often have catch pockets of water and will explode from the heat. If you run into trouble, please do email us and we'll try to help you figure things out.