The most imp thing is, "Decide to be happy in every condition."
Even if you don't feel like being happy then pretend to be happy, try to think about things that make u happy because you concentrate on what you think and you attract what you concentrate.
Stop overthinking, stop negative self talk and stop dwelling your past. Remember that Positivity has a power.
Happiness is very precious, Don't give the control of your happiness to anyone or anything.
Don't expect anything from anyone.
Don't worry about what others will think about you.
Love yourself.
Moments won't come back. So ,enjoy each and every moment.
Up to this whatever I have written is Philosophical point of view.
Now, I am writing some experiences and some things that you should try once.
Make a surprise visit to your grandparents home and spend some time with them.
Cook something for your mummy some day.
Have a lovely fight with your siblings.
Go terrace on midnight, see the stars and just lost in them.
Take a long walk with your best friend.
Gossip with friends till night.
Close your eyes and listen a music you love.
Have a healthy group discussion on some topic.
Go on a bike ride with friends.(I went for cycle ride actually )
Be crazy sometimes.
Give a surprise call to your old friend.
PS: Sorry for such a boring answer and I know it is always "EASIER SAID THAN DONE".
So please do share little things which make u happy!!!