With this painting technique, it is important that you change the water often. This will keep your colours nice and bright. You can also experiment with different brush sizes. Watch how the different brushes react. Or change the amount of water with each colour. The thicker the piece of paper is, the more watery you can make the paint without the paper crinkling.
Ask your parents for a frame or passe-partout and turn your picture into a real little work of art when you're finished.
Please note that paints can stain. Use an underlay and protect your clothes. Spots of paint should be washed off right away with lukewarm water and soap.