Hi Alex,
There is a clear point of interest between the giant and the fairy thanks to their look play. But be careful because you treat the second plan like the first plan (same amount of details). This make the watcher confused about the real point of interest. In a way, put some hierarchy into your storytelling. Try to make it more simple and add the beginning of the atmospheric perspective (some kind of a slight fog).
There are too much different color in you color palette, as a beginner, stick to 3 colors maximum, because for each color you have to deal with the lighting and the more colors and the more lighting skills it requires.
About the lighting, since we only see one lighting source (the sky) you should only rely on this to impact the scene color. And as a beginner it will be easier for you to manage only one light source.
You did good for the background (giant and big trees), using a gradient to contrast with the second plan. Use a bit of this principle for you second plan and this should be already more readable.
One last advice, since you are on this piece for a long time, if you really want to apply the advice the community gives you, don't spend too much time on this anyway. You will learn to let go and start fresh with all you learned rather than adding and getting more lost in the end.
I hope my advice will help you 
