Volunteer man, seriously so many people out there need help. Life can be a real pain, and mostly it's cause we don't help each other out enough. If everyone cared about each other and helped each other out, we would live in a utopia.
Helping others can actually give meaning to you life, and actual happiness. When other people are happy, it can help you feel it as well, no matter how depressing your life is.
Kids especially have a joy of life that is infectious, try tutoring of volunteering in a classroom. Teachers freaking need the help especially in states like California.
You also will make contacts, friends etc while doing all that, which can also help make your life better and more rewarding and give you a variety of activities you look forward to. Try learning to cook if you don't already and then start taking turns making meals with friends or start a local pot luck etc, it's rewarding to make a good meal for others, that everyone enjoys and just as much fun to visit others and have a good meal with them.
Look into playing dungeons and dragons or a card game like magic the gathering with people in your area, and consider using meetup.com to find other activities you might enjoy as well.
I know it sounds silly, but seriously consider joining a local YMCA or other fitness center with water areobics classes, they can be a great way to both excersize and meet people/socialize.
Consider trying to make plans for future trips to do things you might enjoy like white water rafting etc, something to look forward to, that you are working towards. Having goals is also a good way to give life meaning.
Also, if you don't read, reading good books is a great way to break depression etc, there are tons of people on this board that will recommend many great authors if your interested (including me).
Don't worry about being a contribuation to society either, work towards actually enjoying your life and then, once you have ways to accomplish that, you will have the energy and ability to spread that happiness to others around you. Volunteering can help get you there at first if you have absolutely nothing that interests you in life anymore, but long term you need to find your own happiness before you can really start to spread it to others at least long term.
That at least has been my experience.
Best of luck to you:)
Thanks for the advice, but almost all of that is not within my reach right now (since I don't even have a car, and I honestly don't want one, since I would likely get myself or somebody else killed).