Answer:
What is probably the best way to build a model windmill for a garden is by using some plans. (You didn't specify wood or metal.) If you're handy, you might be able to draw some up yourself, or with the help of some family member or friend who has some knowledge and/or experience. Shop around on the net and see what you want (or find something close to it) and get out your paper, pencils and ruler. If you haven't cruised the web looking for ideas yet, start now. No judgment is passed on anyone, but if you are not as good at web searches as you'd like to be, why not practice with this one? It'll help you refine search arguments. And almost everyone has a family member or friend who has a fair amount of skill with the computer. Not many high school kids can't use them, and use them pretty darn well.
There may be back issues of Popular Mechanics or the like with a project piece already set up, but your local library will have the scoop on that. If you haven't been there lately, you're missing out. The library is modernizing, and they can connect to stuff electronically because of site licenses and institution subscriptions to things you'd have to pay for at home. That card that's been getting dusty in your wallet is still worth something!
If you're lucky enough to be close to an "uptown" woodworking shop, get in there and start asking around. (Take a big bag of M&M's and see what you can buy with it. You may be surprised.) It's a cinch someone at a lumber yard can offer some solid suggestions. That's a lumber yard and not a big box builder's supply. Certainly there might be someone who can offer something of value at a big box, but the smart money is on the lumber yard as a first stop. But don't rule anyone out. If you want to do this project, hook up with any and all individuals, groups or organizations that can, or may, help you.
Use the links to get you started. Good luck.