As Freya said, practise with the proportions. Even if it's a chibi, some parts like the hands, or the shoulders, need to follow certain proportions. Drawing chibis is harder than usually thought, and practising with both realistic and manga styles really helps you to improve.
Another thing: if you have to draw straight lines, use a ruler xD It may seem obvious, but the difference of using it or not is huge, specially when you're a beginner. By the way, did you scan it directly from the pencil drawing? I would recommend doing the lineart before scanning, and make the lines thicker or thinner d