I'm sad. How can I be happy?
There are many things you could do to improve your mood, but the most important one is talking with someone about it and trying to find out why you're feeling that way. Do not underestimate how much of a difference someone else's words or actions make on people in general as well as yourself.
The second thing would probably be getting some exercise which will increase endorphins production (happiness hormones) and help get rid of the physical pain caused by sadness.