I downloaded 2020 turbo tax product. I am ready to start my return. How do I start? where is it.

Not sure where you are stuck ... once you have downloaded the program and updated it you should be able to start a new return.  Look in the menu bar and click on FILE. 

Windows or Mac?  After you downloaded it did you install it and activate it with the License Code?

How to install on Mac

https://ttlc.intuit.com/community/installing/help/how-do-i-install-the-turbotax-software-for-mac/00/26067

How to install Turbo Tax on Windows

https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-refund/install-turbotax-windows/L8elzcgsp_US_en_US 

See this TurboTax support FAQ for finding the download for Windows - https://ttlc.intuit.com/turbotax-support/en-us/help-article/product-delivery/find-turbotax-windows-software-download/L1zwlyyvc_US_en_US?uid=l9j3pdpg

See this TurboTax support FAQ for finding the download for a Mac - https://ttlc.intuit.com/turbotax-support/en-us/help-article/product-delivery/find-turbotax-mac-software-download/L6RTW5hbF_US_en_US?uid=l9j3r518

once you download the program, you have to install it. in Windows by double-clicking on the file which should be in your download folder.