Made a captial gain on a second home that is owned jointly, how do I show 1/2 of the gain on my return

You enter 1/2 of the figures for everything is all. All the way down the line. Is this rental property? Or something else? Are you related to the joint owner and possibly filing a joint tax return with them because they're your spouse?

UPDATED FOR TAX YEAR 2019

The sale of a second home is considered the sale of an investment.  As Carl indicated, enter your half of the figures for the transaction.

You can report the 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 Second Home 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.

[Edited | 4/14/2020 |  8:51am PDT]

You are assuming the investment is not rental property. If it is rental property and has been reported on SCH E of a tax return, then the sale/disposition of that rental property "MUST" be shown on the SCH E, regardless of where you may report the sale.