At what point do I need to set up a business?

I am planning on starting to offer my IT services as a consultant/contractor for both consumers and business in my town. I am planning on doing this as a type of second source of income while I am in school to get my MIS degree. I enjoy doing this and want to keep my experience up. My concern is this, while I realize that it is probably safest to start business (actual business entity) from the get go, I am not sure how well this is going to work and do not want to spend the time and money to set up, say an LLC if it is not going to be doable (with my schedule, etc.). I would like to see how it goes before I would set up a business. Is there a certain threshold that I need to meet before I have to set up an entity? I know that on n independent contractor basis, you do not have to claim until you make $600. And if this is the case, is this $600 gross, or $600 net? I am not trying to get away with anything, just want to know what I really have to do and when. By the way, I am in VA.