Where do I enter form 1120-S

You would not enter a 1120-S form into your tax return.  Form 1120-S is a tax return for an S-corporation and would be filed separately.  

However, if you are a shareholder in the S-corporation, you would be issued a Schedule K-1 (for your federal and state return if applicable) that is then used to report any applicable income in your personal tax return.

To report your Schedule K-1 in your personal tax return, log back into TurboTax.

1. Select Income & Expenses.
2. Scroll down to the section titled S-corps, Partnerships, and Trusts and select Start to the right of Schedule K-1.
3. On the screen titled "Tell us about your Schedules K-1," select start to the right of S corporations.
4. Proceed to enter the information from your Schedule K-1 into the program as prompted.

Please note, if you have a state return, you might need to make entries into the state interview section if any amounts on your state K-1 numbers differ from the federal amounts.