Honestly, a question like this is difficult to answer because everyone has different things that make them happy. For some people a happy life means finding true love, getting married, and having lots of children. For others a happy life is studying hard and being successful in their chosen career.
For a long time, I personally wanted nothing more than to party as hard as I could, get wasted on as many drugs as possible, and go to raves with my best friends (and before anyone can say differently, for a long time this actually made me very happy even if it eventually didn't end well). The most I can offer you is a list of some things I learned about happiness in my short time on Earth. Maybe some of this will resonate with you, while others of this list might not. I'll write as much as I can now, and then update my list as I think of more things.
Practice calm abiding. Don't sweat the small stuff. Let it pass over you. If you can do this, you are already 90% of the way there since life is almost all small stuff.
Be patient with others, slow to anger, and quick to forgive. Holding onto anger only disadvantages you.
Genuinely listen when someone is talking to you. Take interest and smile at them as they speak. A smiling face puts people at ease and helps them open up to you.
Doing things for others is far more rewarding than having things done for you.
Being rich of spirit is far more important than being rich of wallet, but you should NEVER neglect your material well-being. It's hard to be happy when you are worried about paying rent, or worse, wondering where your next meal is coming from.
Cultivate a genuine love of learning, but always be skeptical about everything you let in your head. You should be open minded, but not so open minded that your brain falls out.
We live in a wonderfully fascinating universe with endless things to explore. Never be scared to explore it.
Do not become a slave to anything or anyone. As I mentioned earlier, I used to do lots of drugs. Eventually my habit enslaved me. I've been clean for five years and have never been happier.
Understand that everything in life is temporary and transitory so enjoy it while it's around. For every person you meet, there will come a last moment where you will never see them again and for most of these people, you will never know when that moment has come until it has passed.
Exercise. Your body is a living thing and needs regular maintenance. Also, don't forget to brush your teeth. Toothaches suck.
Try to figure out what makes other people tick. It will help you understand yourself and also make you a better friend.
Understand that evil doesn't exist empirically. That which seems to be evil is always caused by sickness of the mind, brain, body, and/or heart.
Learn mathematics. It is the language of the universe and the feeling you get when you start seeing the big picture for the first time (which for me happened right around Calculus 2) is breath taking. Plus, mathematics helps train your brain to think logically.
Try to make money doing something you love. If you can't do that, don't worry, most people can't either. Whatever job you get, work hard at it and try for promotions. It's almost always better to be the boss.
If you are someone's boss, treat them fairly and with respect.
If you are young, spend time with people much older than you. Their wisdom is valuable and they have great stories. If you are old, spend time with people much younger than you. There is little so beautiful as the vibrancy of youth. It's contagious.
If you are generally well adjusted, consider trying psychedelics once in life. There is a reason so many religions have used them. If you do, understand that a good experience is all about being in a comfortable, safe place with good people.
Don't worry too much about what others think of you. Instead, focus on being the best you can be and the rest tends to take care of itself.
Be kind to animals and nature.
Don't fall into the trap of comparing your inner foibles to other people's highlight reel. With platforms like Facebook, it's very easy to accidentally do just that. Remember that people on Facebook only post the things they want you to see.
Always thank others for what they have provided you.
If you were born into privilege, understand that there is nothing wrong with this. A lot of people will want you to feel guilty about your luck. Don't. But also understand just how lucky you actually are. You could just as easily been born in some impoverished, war torn country. Or to junkie parents. Or any other difficult situation. Understand that not everyone has the privilege you enjoy. For this reason, don't forget to give understanding and a hand up to people not as lucky as you.