It's hard to grasp how to shade by hand at first, so I started out by using shading templates. I would go on needcoolshoes and make the skin I wanted entirely unshaded, and then I would put it in paint.net and overlay a shading template. It gives you a good idea of where the shadows need to go, and eventually you can develop your own way of shading and play around with colors :)
Side note by all means I am not one of the good skinners on this website lmao but I'm still starting out kinda like you lol. It also helped me to look at real life reference pictures and try to recreate that as close as possible