Don't panic. This is very common and very manageable. Here's what you need to know:
1. Self-employment tax. On that $8,500 you owe 15.3% SE tax (Social Security + Medicare). That's roughly $1,200. This is on TOP of regular income tax. It's the part that surprises people.
2. Income tax. The $8,500 gets added to your W2 income. At $61k W2 + $8.5k freelance, you're in the 22% bracket. So roughly $1,870 in federal income tax on the side income (after the SE tax deduction).
3. Total tax bill: Roughly $3,000-$3,100 depending on deductions. Set this aside NOW.
4. Schedule C. You'll file this with your regular 1040. It's where you report the freelance income and can deduct business expenses (software, equipment, portion of home office, etc.).
5. Quarterly estimates. Yes, technically you should have been paying quarterly (Apr 15, Jun 15, Sep 15, Jan 15). Since you missed them, you may owe a small underpayment penalty -- usually $50-100. Not the end of the world.
6. Going forward: Use IRS Form 1040-ES to calculate and pay quarterly estimates. Or have your W2 employer withhold extra to cover the side income (simpler, and avoids quarterly payments).
You can deduct half the SE tax from your gross income, which helps a bit. And definitely track all business expenses -- they reduce your taxable freelance income.