Summarize:

It's the icon with Windows logo.  By default, it's in the bottom-left corner of the taskbar.  This opens the Start menu. You can also press the ⊞ Win key to open the Start menu. This displays the Command Prompt icon at the top of the Start menu.  As you type, a list of programs and search results appears in the Windows Start menu. It's the black square icon with a small white "C:\" in the upper-right corner. This is the Command Prompt command to check which version of Java you are using.  Type it in the Command Prompt window. This displays a readout of the current Java version that is installed on your computer in the Command Prompt.  Make sure you include a space after "java".  If it doesn't display any information or there is an error while running the command, you might not have Java installed on your computer. To download Java, go to https://www.java.com/en/ in a web browser and click Free Java Download.  After you install Java, restart your computer. If you have Java, make sure you have the latest version of Java. To determine if your system is running a 32-bit or 64-bit version, read How to Check If Your PC is Running the 32-bit or 64-bit Version of Windows to determine which version of Windows you are running.
Click the Windows Start  icon. Type cmd. Click the Command Prompt icon . Type java -version in the Command Prompt. Press ↵ Enter.