From the IRS:
How do I know if I have to file quarterly individual estimated tax payments?
Generally, you must make estimated tax payments for the current tax
year if both of the following apply:
- You expect to owe at least $1,000 in tax for the current tax year
after subtracting your withholding and refundable credits.
- You expect your withholding and refundable credits to be less than the smaller of:
- 90% of the tax to be shown on your current year’s tax return, or
- 100% of the tax shown on your prior year’s tax return. (Your prior year tax return must cover all 12 months.)
There are special rules for:
- Farmers and fishermen
- Certain household employers
- Certain higher income taxpayers
- Nonresident aliens
Estimated tax payments are a way to ensure that you don't incur an underpayment penalty when you file your annual tax return. Since you already have a "W-2" job you most likely have taxes withheld through that, which might cover the 90% of current-year tax or 100% of last year's tax due.
If you aren't able to make those calculations yourself and/or don't want to spend money on a tax advisor, you could just send 25% of your extra income as an estimated payment - at worst you'll get it back as a refund when you file your annual taxes.
As far as TurboTax goes, it should ask you a question at some point in the interview about estimated tax payments. If it doesn't (or you didn't recognize the question properly), look under Estimated and Other Income Taxes Paid in the outline.