I think the more important question is how can you learn not to be sad. Once you do that, happiness will follow. Here are some things I do:
When a problem comes up, I don't care how terrible it is, I immediately start thinking of possible solutions. An example would be when my car broke down. I didn't cry that I had to spend money, I started making a plan for how to get to work without my car.
When something is out of my control, I accept it, adapt to it, and move on. That doesn't mean ignore it or forget it. My dog was getting old. she passed away. It's not cool to keep my feelings inside so I cried for months when I missed her. I adapted my schedule (no more breaks and walks because he was my only dog), but I accept that it's okay to grieve. I have moved on.
I don't let myself get "bored" often. That doesn't mean I'm always doing something, because sometimes I just enjoy lying on the couch and looking up at the ceiling and thinking. But it's not boring. It's thinking. Plus, it can also mean taking the time to do what you want. My husband likes it when I see him playing video games so I often do. But sometimes I want to do my own thing and he understands.
Find a hobby you like and take time for yourself to enjoy it. do you like painting? Study? Video game? to Cook? People rarely turn a blind eye when you spend money on books or new pots and pans, but if video games are your thing, don't feel bad about investing in your hobby by getting that latest DLC. Just be careful and reasonable about your spending as it is easy to spend large amounts of money when all you have to do is hold your finger on your phone.
Keep your house clean. A clean house can only help with a clean and healthy mind. It is a low stress one.
I had a prior complaint about how I "can't wake up happy" like her. Sorry, but I just don't. I don't even get sad. I am neutral The events of the day affect my mood in some way or the other. Best wishes!