I misread that, I thought you were saying you had 15 now and would have 18 new ones in the spring. You were clear, 18 total, it was me misreading it. In Virginia they should pretty much have access to the outside all year. That helps a lot. And I thought you already had a shed. If you get a shed from a big box store you can put it together yourself and you have to provide a foundation but they can get expensive.
The simplest way would be to buy one of those kits and build it yourself or pay someone to assemble it. You can get metal or wood, either can work. To turn it into a coop you need to add nests, roosts, a pop door, and ventilation. Doing that isn't that hard if you know the techniques. Hopefully we can help you with that. You may need to buy some tools though.
Another possibility is to find someone giving away a shed on Craigslist. Usually you have to tear it down and transport it yourself. That can be a lot cheaper but involves a lot of work. You probably can't transport the size you need unless it is disassembled.
The minimum size I'd want for 18 chickens is in the range of an 8' x 10' coop. That gives you enough room for nests, roosts, food and water inside, room for the chickens to not be overcrowded, room for you to move and work in there, and leaves you some room in case you want or need to add something inside. Stuff happens and I really like having the flexibility to be able to do things if I need to. I'm thinking of stuff like a broody buster, a brooder, a place to isolate a broody hen or an injured hen.
If I were building one from scratch I'd probably build an 8' x 12'. Most building materials come in 4' or 8' dimensions in the least expensive sizes. If you plan for that you can reduce both cutting and waste.
For plans you can look in the coops section as already mentioned. But you are basically just building a shed. You might find books at Home Depot or Lowe's that tell you how to do that. Your local librarian may be able to help you find books you can check out, ether to use to build it or to see if you want to buy it.
I'm not a cabinet maker or any kind of fine carpenter. But what we are talking about is rough carpentry. You can handle that.