If you don't already sew, I'd start by making a pillowcase or a dice bag, and then maybe a shirt or something. Take some time to learn about fabric materials and weaves. Pants are notoriously hard to fit, and if you're also learning to use a sewing machine at the same time, it's gonna end in tears. Or even tears.

Youtube is chock-full of "learn to sew" videos, and you can buy paper patterns online, or in person (and get helpful advice) at your local fabric store.

The instructions that come with such patterns are bad, but still better than the average readme.

Then when you've got a feel for it, you can start drafting your own patterns. Much of the information that's available on that is for womenswear, but it doesn't take too much thought to translate things.