i am in the process of filling and finishing a 12x24 work shop/shed. first decide what you are going to do. if blacksmithing, forging or using a gas burning furnace, i would want a concrete floor. my shop is for stock removal knives, woodworking, and electrical work. i have the inside perimeter is either work bench or drawers. i have a dedicated bench for drill press, 2x72, 4x36 and 8"disc sanders, sliding compound miter saw, table saw, and band saw. get all the drawers and shelves you can find. search your local are for reclaimed or recycled building supplies. you can find old kitchen or bathroom cabinets for a lot less than just the raw materials to do it yourself.
a one car garage would work. a 20 to 30amp 240vac outlet is a must. find power strips with built in circuit breakers on sale somewhere and buy a half dozen. start rounding up any empty plastic containers with or without lids. silverware organizers by the dozen work well. look at what you use every day vice once a month. i have space under the sliding compound miter saw for a second miter saw. my planer(sorta portable, makes such a mess i only use it outside) rests in a roll around box that fits under the drill press table. router table sits on top of shelving unit(not used every day).