How do I download Yosemite on my imac?

I have an iMac (newer than 2007),and I need to run this year's version of Turbo Tax. I am getting a message indicating that my current OS X, which is 10.6.8, is not sufficient to run this year's Turbo Tax (premier version).

I have tried to download Yosemite and have been unsuccessful. I assume that once I download a new OS, that it will appear in place of the 10.6.8 when I click on the Apple icon at the top left side of my screen and ten click on about this mac. How can I download Yosemite?