No one has an absolute answer. Just make a purpose. That's all I've done. Actually, I technically haven't even done that as I have made my purpose... to find purpose. Life is just one big experience. Take it for what it's worth, don't regret anything, die in 60 years and then, fertilize a tree or something.

Although probably the first place you should start with asking yourself why you're here is, do you believe in a god. Maybe if you told all of us that, we could actually help you, as gods have funny ways of giving people purpose. If you don't believe in a god, then see my above paragraph about fertilizing trees, because that's all the advice I can give you at that point.