For Miku (I'm assuming you mean Hatsune Miku): Break down her cosplay into pieces. Pleated skirt, sleeveless shirt, tie, boot covers/thigh-highs, and arm socks, as well as all those little accessories and a wig. You need a budget, or at least a limit to how much money you want to spend. The amount of money you are planning on spending will dictate what kind of fabric you buy. I'll break it down. I'm also going to assume you have a Joann's Fabrics or another similar store nearby.
Cheap- Broadcloth and cotton. Broadcloth is highly warned against, since it is thin and you will easily chafe. I made the mistake of making a Godtier Rose Lalonde cosplay out of this stuff, my first time sewing a cosplay. Nope. My neck and underarms were all red. Don't use it. It will cause pain. Cotton is cheaper than some other fabrics but you might chafe if you have sensitive skin. Also it wrinkles, and you usually have to buy it all at one time because there are different dye lots and the colors will be different if you don't buy from the same bolt. It looks pretty nice, but requires a lot of ironing and attention. It might be best to make the skirt out of it because it'll hold the pleats better than what I'm going to recommend next...
Middle price- I love love love love crepe. It's soft and durable and looks nice on camera (from far away, that is. Really close up it looks hideous, like a pilled knit). I made almost my entire Madoka cosplay out of it, and it holds up so well. Also, since you're making a Vocaloid cosplay, I'm going to recommend something you're not supposed to use: satin. That's right, shiny satin. Vocaloid cosplays are the few cosplays you can actually pull of with satin, because they have that shiny *star quality* to them. You don't have to use satin though. People might make fun of you.
Expensive: Crepe-back satin or a PVC-polyester blend (AKA pleather). Both are shiny.
TL;DR: Use the crepe. Its my husband.
Also, go to the fabric store and touch all of the fabric. Roll out a few bolts and compare them. Take a picture of it with the flash on and off. See how it looks on camera. Check to see if you can iron it (ask an employee, I actually don't know that much about fabric).
For making the costume:
Look for patterns. They are basically tissue paper stuffed inside little envelopes. You can "edit" patterns if you figure out how they work. I.E. find a pattern for a blouse, but instead of putting sleeves on it, use bias tape to finish of the armholes. Make a pleated skirt shorter and trim it with teal/turquoise/aqua/whatever color ribbon. As for the armsocks and bootcovers, Google is your best friend. Making your own pattern is super easy once you learn how a pattern works.
I've never made a Miku costume, but breaking it down it's very doable. Gather all the reference pictures you can or download MMD and look at the Miku model!
Don't be afraid to ask specific questions! Good luck!
Tanaka Ryuunosuke- 10%
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Eli Ayase- 5%
Winry Rockbell- 95%