since it's become so common, they just created a "Gig Economy Tax Center."
because the IRS considers income taxes to be a "pay as you go" tax. this means if you don't "pay as you go" (as in you're an independent contractor and no taxes are withheld from your paychecks), you're expected to estimate what your taxes will be and make quarterly payments.
if you don't, they can assess a fine on top of the taxes you owe, even if you file in a timely manner. it's kind of a messy pain in the ass to figure out what the penalty is.
incidentally, this is why rich people pay tax experts to manage their tax shit for them, and why it's easier to stay rich (or get richer) when you're rich, and why it's easy to get poorer when you're poor.
anyway, if you're in the US and this is relevant, here's the link: https://www.irs.gov/businesses/gig-economy-tax-center