What are some of your favorite things that improve your quality of life (under 20)?
There are myriads of possible answers here, but I'll try. First, accept yourself. Do you believe you are ugly, fat, short, bad student or whatever? Accept it or change it. If you cannot change it (height), then accept it. If you can change it (weight, grades), try. If you are bored, then face the consequences of that by admitting it. Self judgement and improvement will really improve your life quality.
Then, find subjects and do projects that interest you. It's very satisfying to believe you are good on something.
Money is important, but it's not a necessary condition to have a good life. Learn how to stay satisfied of what you've got in the short run.
Flirt and have sex. Healthy sexual relationships increase the quality of life in the long run, as it increases your confidence and social relations.