Happiness is something created from within, shaped of your thoughts and view on life. With that said, YOU are responsible for your own happiness and no one else can make a change, except you.
What you think, you become. I've mention this quote by Buddha before, but it's basically all you have to understand. You can't live a positive life with a negative mind, so get rid of the thoughts that doesn't make you any good. Thinking positive is something you'll have to train your brain into, practice on repeating positive statements to yourself and after a while you'll create a state of happiness, because your attitude becomes naturally more joyful and positive. That leads us to next point...
Turn everything bad into something good. I started doing this a few years ago and can clearly say it had a big impact on my life. I believe that everything happens for a reason, and that it's really important to be able to see the positive in every situation. I remember coming to university one day and the elevator was broken, I had to climb eight stairs in order to get to my classroom but was I mad? Nope, I said to myself "great to get some exercise in the morning" and climbed the stairs with a smile on my lips. You can actually trick your brain, and before you even realize it you'll find yourself in this virtuous circle of happiness.
Don't care what other people think. I personally know that this is easier said than done, and it took me many years before I could accept myself and live my life after my own conditions. Someone once said "one of the greatest mental freedom is truly not caring what anyone else thinks of you" and it's not until now I actually understand the meaning of these valuable words. Many people grow up focusing on what other people or society think they should do, without discovering who they really are. What do you love, and who would you be without the influence of others? This is your life, be yourself, focus on your own dreams and the right people will come to you. People who can't accept you for who you are don't belong in your life anyway.
Comparison is the thief of joy. Comparing yourself to others can not only be really tiring but also slowly break you down in the long term. There's no one in the whole world like you, so embrace yourself and make the best of what you got. Every person has their own strengths and weaknesses, and you have no idea what their journey truly is all about - so comparing yourself to them is just a waste of time. The only person you should compare yourself to is the one you were yesterday, and always strive to be a better you.
Stop making excuses. Want to work out but don't have time? Get up one hour early and get it done. There's always a solution to every problem, and if you can't find that solution, it's probably not that important to you anyway. Only you are in charge of your life, and no one else can make it work for you.
Doing what you like is freedom, liking what you do is happiness. Go after your passion, and devote yourself to something that makes you want to jump out of bed in the morning. Work is going to take up a huge part of your life and if you don't feel satisfied or genuinely happy with what you're doing, maybe it's time to change the road you're on. Life's too short.
Respect your boundaries and people will respect you. We can spend incredible much time on pleasing others, and although it's important to make people happy, your feelings has to come first. I've come to realize that when you stand up for yourself and learn how to say no, without explanations or apologies, you become a lot more happier within yourself. Practice building self awareness, discover what you are willing to do and what you're not. If you don't feel comfortable in doing something, let them know. They might seem mad at first but eventually will come to respect you, so it's a win-win situation.