It may be before expenses... can't remember! Ask me again, say, mid-April!
You don't really have to do anything extra special to file a schedule C. You just fill out the form, give it a name (which is really just a reference name - it isn't a public name or DBA or anything, so it doesn't have to be cute, just descriptive. "Teaching saxophone" or "painting" or "VCR repair") and fill in the boxes for how much money you brought in and how much you spent on it. It's not like forming a corporation or partnership or getting a business license or anything like that.
It's also completely independent of licensing or zoning requirements for the specific type of business in your area. The IRS does not care if I am manufacturing doodads in my house when my city requires an industrial-zoned site for doodad manufacture. They just care about the income.
For various reasons, it might be advantageous to have a corporation or partnership. For example, an LLC will protect your personal assets if someone decides to sue your business. And those structures become important if you have a business partner or co-owner, sometimes if you have employees, possibly to take advantage of various grants or tax incentives, and sometimes if your industry involves purchasing from wholesalers who want to only sell to legit businesses or whatever. But you don't *have* to do that, and if you're a one-person business with a low risk of being sued and there aren't any of those other incentives to take advantage of, there's no reason you ever have to form a corporation or partnership.