I'm having trouble with the details. What should I do to improve?

Let's start with the background. People often make the mistake of drawing the character first, which makes it hard to fit them to the background. It's easier to draw the background first, then draw the character. Details can be fixed by practice. I recommend making a little box in the corner of your paper/digital art device and making a little model of the area. When you're done, you can sketch or blow up the whole picture. You can then do line art/inking. Make sure you don't leave any blank/boring spaces or it will make the viewer look at that instead of the actual piece.