Hi Wenruto,
Consider carefully the composition of the piece. Sure, this began as a doodle; the meanderings of a pen on the page. But if you want to take the time to translate it into an art piece, I suggest thinking about how you want to organise it, what you want it to focus on, etc.
Work iteratively across the entire piece. Block in the composition with extremely basic lines/shapes and then refine these across the entire piece repeatedly until you are satisfied it is finished. This method, as opposed to working on where ever your interest/focus leads you, will ensure you catch problems in composition/structure, proportion/scale before serious reworking is required to fix them. It also makes setting aside and returning to a piece a lot less troublesome and increases the ease of analysing and critiquing.
Use basic shades of grey for your blocking in process. Colour consideration can come later on, since shade value is far more significant and hues can get you caught up in constant tweaking/rebalancing continuously as you work. Also, working in greyscale lets you easily judge the balance of the composition.
Be sure to eliminate jagged lines. They dominate your piece currently.
Avoid gradient-shading. It contributes nothing to the rendering of an object. Check out the
Tools, Resources and Linkage Thread (or search the net) for tutorials and information on light and shading.
Having said all that, I like the idea for the piece. It has numerous (in my opinion) extraneous elements that should be culled. The furry thing hitting the ball and the pipe pouring out water being two such things. Just my personal take.
EDIT: Oh also, select colours that work in harmony with eachother. You have extremely saturated (to the point of neon) colours at the moment. Work with a palette of related colours that can blend in to each other harmoniously. This requires much more subtle use of saturation.
