How do I qualify for the grocery credit?

It depends on your state, but TurboTax will let you know the requirements for the credit when you complete your state tax return if your state has such a credit.  

The federal government does not offer a tax credit for food purchases, but a limited number of state programs offer grocery tax credits for individuals and certain businesses. Each grocery tax program has its own guidelines and your eligibility may vary. Certain states offer grocery tax credits for all taxpayers, while others extend eligibility only if you meet income limits. Depending on the state, a program may exclude you if you receive certain forms of public assistance or may offer higher credits if you are a senior citizen.