The basic information that you need in order to search information on the internet about any software, is their version.
Knowing the version helps you to organize your learning process on a specific tool and to track possible bugs and new features that you find along the way.
If you need to get the information of the current installed Sitecore version, go to the login page, and check the version on the right side. But if you are using a Sitecore version higher than 8, this will be a slightly different.
The Sitecore 8 has a better looking and you see the login with a red background like the picture below.
After you Login into the Sitecore, you will see a bunch of buttons if are an administrator, or just a few if you are not. hit “Control Panel”
After that, just find the “Administration” and “License Details.
There is you Sitecore version with the revision build
Sitecore Tips 