for folds and wrinkles, go for thiner lines and choose another color than black. Also keep in mind that you can't throw random lines on a shirt and call them folds, it'll just feel look off. Folds tend to point in the same direction, towards a point on which tension is applied (elbow, armpit, ankles etc.).
Your shading is simplistic but not too shocking.
Anyway it's not the kind of thing you can guess, I also have these problems. You should look for references or find tutorials to improve yourself.
Looks to me like you nailed pretty much everything else, nice job