well u think about everyone u care about and the things that make u laugh or smile then hold that in ur mind for a while and then ur happy but if u mean it in a different sense the answer is:
remove any doubt from u mind that makes u angry and remember the good things and don't let sadness over take u but the thing is u need to be able to let it out when ur sad but by writing it down in a journal so that it is safe and it will calm u down and untimely lay the bricks down for a happier life