Depends on location city or rural what kind of shop, production or job shop, and the machines you have or can afford. A sheet metal shop is probably a safer way to start, all the machines can be picked up cheap the customer does all the design so if you can read a tape you never have to eat any cost. Use that to buy a lathe mill 5 axis etc and pick up other kinds of work as your network grows. Construction jobs need lots of simple steel parts, plate steel with holes, beams, pit ladders custom brackets and so on.