One of the first things I have got to say about this is that your box is off. It is a good bit shorter on the right than the left. Although panels like this are used in comics they are reserved for dynamic scenes, action, shock value or important events.
I mention this because I'm sure you didn't mean for this to be intentional. A tip for the future; Start in the upper left corner of your page. Measure from the top of your page down to where you want to top of your panel/drawing to be and make a mark. Do this again from the top of the page 2-3 more times a few inches apart so you can connect the marks with your ruler/straightedge.
Now do the same thing, but use the left side of the page this time. Measure in to where you want to make your left edge of the panel/drawing. From there you can measure how width and height of the rest of your box to make sure all the edges are even.
I know it seems like a lot of work, but it is really helpful of you don't have a drafting triangle. The easier solution though is to just buy a drafting triangle.
You'll pick up on that as you progress.
I really don't like the line work in this. Mostly for two reasons, it has little to no line weight, and its but splotchy and not black.
This is where you will want to do a little post work in your program before you start coloring. Playing with some of the Levels will help you achieve a blacker more solid line. Thus giving more impact to the piece over all.
Adding some more line weight will increase the dynamic of the picture. Basic explanation. At the moment much of your lines have pretty much a single thickness from start to finish. To have more appeal you want it to taper a little at the beginning and get a little wider and more expressive in the middle and on curves and such.
This is one of those things in drawing and illustration that will take a little time to truly understand, but a long time to truly master. But once you start to get it figured out you'll be much more pleased with your line work.
Coloring and lighting is, something. Sadly not something great. A lot of your colors are flat, and although that can work with some things, its not working here. Adding more shadows and highlights (actually having highlights that is) will help break up flat coloring.
Your coloring and lighting go hand in hand with this piece. At the moment you have only two levels of lighting and thus 2 levels of each color. For example, you have only two colors of brown for the tree bark, same for the grass, and leaves, and so on. you should try to at least work in at least 3 levels of light, Highlight, base color, and shadow. Having at least 3 levels will really widen your pallet immensely.
The composition of the piece is alright. The actual drawing may need to be refined but things aren't too far off balance.
I would advise some sort of background. Perhaps some clouds or something. Again this is going back the the flat color issues you are having at the moment. This is a link to one of my FAVORITE cloud tutorials of all time:
Its a nice and easy read and even easier to do I don't know if you have a tablet or not, but this tutorial can be done with a mouse and/or track pad, you just have to take your time with it.
Having something in your background will help break up that flat color a little more.
At least your trying, that alone is a good thing. Keep practicing and learning. Good things will come of it.