Actually, you don't. Cream of tartar, or tartaric acid, is (obviously) an acidic ingredient. Acids help the egg white proteins denature and form bonds during the meringue stage. So adding cream of tartar will surely help stabilize and dry out your meringue. But it's not essential. I don't use it now, but I did at first for insurance. I find when I do add cream of tartar my whites are a little too stiff and sturdy. And this can cause hollow shells as the batter doesn't settle on the pan all the way. If you are just beginning, a few drops of lemon juice can help the whites whip up nice peaks. But it won't create too stiff of a meringue.