Thanks for the feedback ^_^ Yeah, I should pay more attention to where I put my folds. I'll make it look more natural for the new version but what do you mean, a crease in between the shoulder parts and the center part? I don't get it :/ I'd like an edit of the pic so I get what you mean, if its ok with you. I'd really appreciate it!
There are two important things to know about the shoulder area, when it comes to clothing.
First, on most shirts, the sleeve is a separate piece of cloth from the rest of the shirt, sewn on along a line running from the elbow to the outside of the shoulder. Look down at your shirt now, and you'll probably see the seam. You don't necessarily have to draw in that seam all the time, but it's important to know it's there. I've highlighted it in blue in the following illustration:
Second and more importantly, there's always some spare cloth under the armpit. That cloth is necessary for the wearer to be able to raise her arm. However, when her arm is lowered (i.e. most of the time,) that extra cloth bunches up and creates folds that radiate out from the armpit. I've highlighted those in red. Again, you don't have to draw in a
lot of folds, but you need at least one to break up the white space and create the illusion of actual cloth hanging over the skin.
Hope that helps!