Buy this book: The Entrepreneur’s Guide to Sewn Product Manufacturing by Kathleen Fasanella, and then register on her forum and you will get your answers and more. You can't get on the forum untill you buy the book, that is how she pays for the forum. But believe me, if you are really interested in sewing your own garments, the book is worth ten times the money.
You can get the book at her site - www.fashion-incubator.com
THIS IS THE BEST BOOK EVER!!!! I've owned it for years and still flip back to it!
Cutting and sewing your own blanks and hoodies can be done, but it is pricey. Most people take a popular silhouette and "knock it off" pattern-wise. With something as simple as a T-shirt or a Hoodie, it's best to buy wholesale. Unless of course, you have high quantities. If that's the case then people will talk to you about getting a better deal than what you usually pay for wholesale.
To start from scratch, you'd have to get a pattern made. Secondly, you'd have to grade that pattern into different sizes. Next comes the prototypes (do they fit right?)...If the prototype needs tweaking (which they usually do), that takes time and more patterning. Once that is completed, it's time to move on to fabric.
You have to pay by piece for cutting of the fabric.
You pay by piece for sewing the fabric.
You usually pay by pound for dyeing the garment (that's if you chose to garment dye, which is most popular).
After all this is complete, you have re-invented the wheel when it comes to t-shirts and hoodies.