Any rules you learn here are made to be broken. With that in mind, take your pencil and draw some sort of circular shape on your paper. You have successfully started your first cartoon. The face is the most important feature, because emotion is always captured there. Now draw eyes within your circular shape.
Eyes can be as big as saucers or just tiny dots.
The nose can be as big or small as you like, depending on how whacked- out you want your cartoon character to be. Add some ears if you like, or you can even leave them off. The mouth really tells all. Turn a frown upside down and the attitude of your character changes completely. Do a mirror study.