From the Credit Karma site, under the More menu, you can click on Taxes to get started with filing your taxes on TurboTax. By clicking the Taxes button, you can create or login to an Intuit account and link it to your Credit Karma account.
Or, from the TurboTax site, you'll have an option to link your Credit Karma and TurboTax account as well after you file your taxes. Please see the FAQ link provided below for further assistance:
Link your TurboTax account to Credit Karma