Forge fuel, that is. You've got three choices, gas, coal or charcoal. They do make electric forges, but they cost a fortune to run very much.
Gas, natural or propane, has definate advantages. It costs no more than coal, you can do anything with it you can with coal, and if you're doing production work- lots of the same thing- you can adjust it to a certain temperature and it'll stay there. You do need a forge designed for gas, many build their own or you can buy one. The one real disadvantage from what I've heard is that once you light it, it has to stay lit, you can't just turn the blower off for a while.
Coal is the standard. Downside is it smokes, it's dirty to mess with, and if you don't get good coal you'll be constantly cleaning out the fire. A friend once gave me fifty pounds of coal that he'd never had a chance to use, and I wound up throwing it away; it was the nastiest-burning stuff I've ever used, and constantly clogged up the fire. Good things are it's easy to get in many areas, it'll keep forever as long as it has some shelter from rain & ice, it burns hot, and you can turn the blower off to take a break, and it'll still be burning when you get back.
Charcoal was the fuel of choice for many centuries. It burns clean and hot, and could be made anywhere there were trees to cut. Nowadays the problem is you need raw charcoal, and when you can find it it's damned expensive. Briquets will work, but the stuff they use as a binder to keep in in nice neat briquets produces a huge amount of ash and clogs things up. And you will use a lot more of it than coal to do the same amount of work. I read once that in Britain Elizabeth 1st issued an edict ordering the iron industry to switch from charcoal to coal: she had been informed that the forests of Britain were disappearing into the furnaces, and there would not be wood left to build ships at the rate things were going(takes a lot of trees to fuel a blast furnace).
If you're in a place where you'll be clearing land and can make charcoal from the timber, you're in good shape while it lasts. If you don't want to mess with coal and can't get raw charcoal in quantity, gas might be the way for you to go. Any of the three will work. I've used charcoal and coal, and prefer coal simply because it costs less than charcoal and I don't want to buy/build a gas unit. I will add that if you're just starting, or want to try it out, you can rig up a coal forge fairly easily, and do good work with it.