We sold an inherited piece of land in India and got some capital gains. The taxes were paid in India. Where do we declare the gains and taxes paid?

You can report the land sale in TurboTax using these steps:

1. Click on Federal > Wages & Income
2. In the Investment Income section click on the Start/Revisit box next to Stocks, Mutual Funds, Bonds, Other.  
3. If you have already entered some investment sales, you will see a screen Here's the investment sales info we have so far.  Click on the Add More Sales link.  
4. If you haven't yet entered some investment sales, you will see a screen Did you sell any investments in 2019?  Click the Yes box.
5. On the screen, Did or will you receive a 1099-B form, mark the No button and click Continue
6. On the screen, Now we’ll walk you through entering your sale details, select Land from the list in the first box.
7. Fill in the rest of the boxes with the sale information.  Be sure to enter the NET proceeds. 
8. If taxes were withheld from the sale, click the box I have more info to enter that I don’t see here.  Enter the taxes in the boxes that appear.
9. Click Continue when done.  
10. Continue through the screens, entering the requested information.

Do I have to pay tax in US also if I already paid tax in India. I have similar situation:

I got inherited property in India worth around than $100,000. The property will be sold under long term gains and taxes immediately paid in India. In this case, do I need to pay any tax in US? Which form do I need to fill for reporting purpose and what documents are needed for US tax filing.

You will report the transaction using the "Stocks, Mutual Funds, Bonds, Other" interview and tell TurboTax that you didn't receive a 1099-B.  Form 8949 and Schedule D will be included in your income tax return.

"Do I have to pay tax in US also if I already paid tax in India."

Maybe, maybe not.  This is one of those "it depends" situations.  The appropriate interview here is "Foreign Taxes" under the "Deductions and Credits" area of TurboTax.  The foreign taxes paid will be reported on Form 1116. 

"what documents are needed for US tax filing"

If you're speaking about documents outside of the IRS's Form and Schedules, something that needs to be sent in with your income tax return, then the answer is "nothing."  Keep every bit of documentation you receive, preferably attached to the paper copy of your income tax return that you print for your own records.