"Good to you" usually refers to how a person treats you. For example, you should only date someone who is good to you! "Good for you" means that it improves your health. For example, too much sugar isn't good for you.

The rice and vegetables I prepared, is good for you to eat. Practicing yoga every day is also good for you. Your mother was good to you when you were young. When you smile, people will treat you better and be good to you.

Another way to say "good for you" is when you are cheering someone who has done something that you approve. If my friend has taken a driving lesson or accomplished something, I often say, "Good for you!"