You'll probably be treated as self-employed, which means you are responsible for handling all the taxes yourself.
When you file your tax return you will include Schedule C. You might want to get a tax accountant to help you the first time; in subsequent years just copy what they did.
For just a few months of 2019, you probably won't earn enough to owe regular income tax. But you will owe self-employment tax (which is the Social Security and Medicare taxes that everyone pays regardless of income level).
You might also owe tax to your state, so look up the filing requirements there.
Tax payments are due in four installments per year, and you can pay online. For now, estimate that you'll owe about 15%. If you've never owed tax before, there won't be a penalty for not paying until you file.
Or if you have a regular job in addition to your freelance work, you can just increase your withholding there.