There isn't a specific place to enter your Form 1099-K in TurboTax. That's because your 1099-K income should already be included in other income, such as your self-employment or rental income.
If you have self-employment or rental income, the amount reported on the 1099-K is reported in the Business Income and Expenses interview or Rental Properties and Royalties interview of TurboTax Self-Employed (or Home & Business).
If you received the 1099-K for online sales of personal property (eBay? PayPal?), you don't need TurboTax Self-Employed (or Home & Business). You can enter it in the Other Reportable Income interview, the very last interview in Wages and Income (Federal Taxes tab > Wages & Income > Less Common Income > Miscellaneous Income). For the Description, enter "1099-K Sales".