Purchase a sketching stylus from an electronics accessories shop. Though there will be several to choose from, select the stylus that closely mimics that size and feel of a pen for the purpose of drawing. It should feel comfortable in your hand.
Make sure you've downloaded a drawing application. To start, you can choose a free or low-end application and then work your way up.
Find a pair of fingerless gloves or cut the fingers off a pair of gloves that you already have. You'll need a glove because the drag of your writing hand across the tablet's touch screen might interfere with your drawing. The glove will keep the touch screen from detecting your hand. With the fingerless glove on, you will also be able to hold the stylus easily without it slipping from your grip.
Touch the stylus to the screen and draw using the application's interface. Try out different colors, brushes, and backgrounds to create a masterpiece.