Now that you've found that perfect sketchbook and gathered your supplies, I want to give you a few simple tips for getting started.
First, you should get to know the colors in your palette and learn how to use your brushes. Start with a blank sheet of paper and pick a medium size brush to begin. Then, start laying in each color of your palette on the paper like I've done above. Wash your brush completely in between each use to keep the colors from mixing.
Label each color swatch with a pen or pencil and continue until you've laid out each color individually (see example in photo 2). Now you can try mixing colors being sure to label each mixture with the colors you used. Try making different marks using the edges of your brush. For some of the color swatches use more water- for some use less, letting your brush go dry.
Play around with your colors and have fun using all the different brushes. Notice that some colors will be transparent (like yellow, purple, grey, and some blues) while others will be bolder with a little more opacity (like green, some blues, red, white). Blues tend to "separate" while they dry and typically colors will fade a bit when they're completely dry. Reds and oranges are stronger colors that want to dominate and grab your attention. When mixing red with other colors you'll notice how a tiny bit goes a long way.
Once all the colors have dried completely, go through each one and put a mark next to your favorite colors. This little exercise will help you "establish a palette" for future use. I tend to use greens, reds, browns, and blues the most (can you tell?).
Now celebrate because you've just completed your first sketchbook exercise! :)