As Ole Rooster said, go to the "Coop Designs" section at the top of this page and look at those coops to get ideas. Here's the link. A lot of those have some nice photos of how they put them together.
https://www.backyardchickens.com/chicken-coop-large.html
Go to the library and look at the books on how to build a shed. You'll find a lot of books with good designs and good details on how to build them. Home Depot and Lowe's often have some books for sale that can be pretty good. The only differences in a shed and a coop are the ventilation, nest boxes, roosts, and a pop hole.
For 20 to 25 chickens, I'd suggest a minimum of 8' x 12' for the chickens area, larger if you want a separate storage area. You can build it as fancy or plain as you like and as suits your area. That's for you and your neighbor's benefit. The chickens don't care ahow fancy it is, but in some neighborhoods you need a decent looking building. If you build it where or in a way that water will not run into it, you can cut costs by using a dirt floor.
There are certain management techniques where you can get by with less space, but you commit yourself to never leaving them locked in there much when they are awake. I like to sleep in occasionally instead of getting up at the crack of dawn every day to let them out and I have a lot less trouble finding a chicken sitter when I go on vacation if they don't have to be there at the crack of dawn. If you live where the weather can be pretty rough, they need a snow-free outside space they can go to in most weather if you build it small. Besides, I find that giving them enough space lets me work a lot less when it comes to poop management and I don't have the chicken aggression issues that tight spaces can give you.
You can get one of those shed kits from Lowe's or Home Depot and put that up, but if you are building from scratch, I'd suggest keeping it in increments of 4' or 8'. Most building materials come on those lengths, so you will have less waste and cutting. An 8' x 12' will probably be cheaper and easier to build than a 10' x 10' and very little more expensive than a 8' x 10' for example.
At that size, I strongly suggest it be tall enough for you to walk in it. You'll need to get to all corners of the coop, either to clean or maybe to retrieve a sick or injured chicken. If you give them a chance, they'll make a nest where you can't get to it.
I suggest you look at something no more than 8' wide, no matter how big you build it. The wider it is, the more expensive the wood to span that width is. 8' is a good width. No matter what type of roof you build, you can find plenty of examples of the sizes of wood you need.
Make sure you have roof overhangs so you can put ventilation openings under there without rain blowing in too bad. Make sure your roof slopes so water runs off, and make sure it slopes so that the rainwater does not run into the run or over a door.