Why are you alive?
Well, when two people bump uglies without any contraceptives, it tends to result in more people. Couple this with the fact that nothing has killed you yet, and *presto*, that's why you're alive.
In all seriousness, what do you enjoy doing? See if you can find a way to make some cash off of that, or just find a way to make it beneficial to someone. Do you have any friends you can talk to about this? Go hang out with them. Talk things over. See if any of them know of job openings you might be qualified for. Possibly talk to your parents about getting psychological help (I know that's the last thing you probably want to hear/do). Try volunteering somewhere (soup kitchen/food pantry, local charity, political event, whatever.) Try writing, drawing, or composing something. Maybe even sculpting or painting. Possibly even exercise (I've got a few friends who started jogging/lifting as a way to cope with depression/nihilistic thoughts. They claim it worked wonders).
Do something to keep yourself engaged with life and other people, or at the very least with your own body and mind. Plus, on the off chance that any of the religions are right about the afterlife, most frown on suicide very highly. Hell, maybe try attending a local church and seek guidance there (I say this as someone who is VERY against organized religion). It does help some people. Just don't start looking for the way out yet.