By request, I wrote a tutorial on how to draw folds. It is one of the things that is often said to be very hard or complicated. Well, it actually isn't true, at least with my method.
I will teach you my folds technique here - you will learn to create folds on a dress.
The really hard part is the correct placement and shape of some folds, in t-shirts or on arms etc.
You can only learn this by studying folds on clothes - how they look, if they are deep or soft, wide or narrow long, curvy, straight...
You have to observe a lot to make good textures.
With that said, on to the tutorial!
You need Photoshop for it.
You can download it here.