My thoughts:

How do you know you are suffering from depression?
Only a professional can help you with that.
How can you turn it around into something positive?
People with certain mental disorders (such as depression and GAD) are often more creative and have interesting perspectives. However, I try not to think of mental illness as a positive, but rather an unavoidable fact. It's neither good or bad, just a part of me that I have to manage.
How can you stop worrying about everything to the point you just feel like jumping in a hole and pulling the sod over you?
Face what scares you. Its trite, but true. This isn't an overnight fix.
How can you find out why you're depressed? Why do you feel like crying but can't? Try to cry. Its a release. Exercise, creating something (art, a meal, etc ...), and nurturing something can also help in a different way.
How can you be depressed and happy at the same time?
I wonder the same. Perhaps its not as abnormal as we think?
Will getting a job take the depression away?
I think that it could help. You get to channel some energy, make money, and have the opportunity to meet new people.