Yeah if you want the deep philosophical answer, you can't technically "make" someone happy, people choose how they want to feel. But you can definitely give them some nudges in the right direction!! Compliments is the generic answer, but it's generic for a reason, it works!
People also enjoy feeling like they are useful, a kind of self-validation, so saying "thank-you" a lot, even to the little things really helps out. Probably the most effective thing you can do though is be a good example. There's two things I've noticed about people over the years, the first being that people are what you make them (so don't bad-mouth people, not even those you don't like), and the second is that people emulate those they have contact with regularly (thus the importance of being a good "happy" example).
Whatever you do though, don't pry into their life, tell them how they should act, try to change who they are, and be careful about getting them gifts (it's easy to send the wrong message). Best of luck!