Well, what's causing you to be unhappy? Once you find that out, try to change it so it's better and then you're happy. If you just feel a little down today, you could draw, listen to music, hang out with friends or family, play with a pet, go watch the sky (just don't stare at the sun!), get some rest, drink something, eat something, etc. I hope you feel happy soon!! <3 <3
Simply count your blessings. Keep in mind that no matter how bad things are, they can always get worse. Look around you at other people who don't have what you've been blessed with. Smile. That's good too.