You can mix a little water and paint on your palette. Paint one of the squares with the color. Then use a little more water when you paint the second square so it will be lighter. Rinse out the brush a little more to paint the third square the lightest color. Practice painting squares of different colors with more or less water when you can.
Watercolors are best when painted very thin.
Try to keep your colors from getting in each others way. The colors will get 'muddy' if too many are mixed together. Watercolors are transparent which means you can see through them like colored glass. Paint a yellow square and then leave it alone to dry totally. Then go back and paint light blue on half of the square.