Okidoki !! This is my first critique ever using this system on DeviantART, hope it will go well xD !
First things first; I like how the shading looks here ! I do guess the lighting source is coming from above, but it would probably be better to emphasis the source by either darkening the shading at some places or adding lighter colors starting from the light source when creating the background.
I know shading can be a pain in the ass at times to do, but if you keep practicing and look at tutorials, I'm pretty sure you'll get the hang of it real fast
Also, like Birdy-Papa said, the tree in the background could use some changes. It could be easily fixed though by adding different shades of brown to the trunk and different shades of green for the leaves (and playing with texture by using different brushes). A good thing to keep in mind while drawing objects/backgrounds/characters is that giving a ''3D feel'' is always winner in a drawing xD. The tree should be affected by the lighting, so we should be able to see some shadows on it too ! Think about how a tree trunk is not flat but rough, with lots of cavities. How would that detail affect the shading? Leaves are, most of the time, largely apart from one anothers because of branches. Would lighting get through the openings? They're all small details that helps giving a drawing some ''oompf'', or just more life to it xD.
Ashley's face is a very good exemple of what I'm speaking about: I really like how you did the shading on her nose area and how it creates a feel of depths. THUMBS UP <3
Goth's face in this is adorable -u- you're definitely getting better at drawing characters in the show's style C': The only thing that would need to be worked on are the arms (also Ashley's arms).
Stock photos are you greatest friends in these occasions xD it's sometimes hard to draw what we see in our head directly, so you should never hesitate to look up for inspiration. Various artists have galleries of stock photos, taking different poses at different angles. And if you can't find the pose you want to draw, then taking a pic of yourself doing the pose can also be of great help (I do that and it's funny HAHAH, especially for faces, oh gawd)
SO, overall, you did a really good job here ! Not everything is perfect BUT it's by doing errors that we learn what to do or not next time we start a drawing. You're getting better at each of your drawings, so I have no doubts that you'll do great on your next one