Before getting into the configuration let's have a brief on FFMpeg

After the configuration it gives u following advantages

1. While writing commands you don't have to specify the path where ffmpeg exists. "<path>/ffmpeg <command>" will be reduced to "<path>/ffmpeg <command>"
2. Commands like "set classpath" won't be necessary
3. U can save your commands as batch file and reuse them. (have a look at other post for batch conversion)
4. one configuration that allows you to access ffmpeg from command line tools like CMD,PowerShell (and other apps working on STDIN/STDOUT mechanisim).

Well, lets step into the configuration

Download FFMPeg

• Download the best build suits for you from official FFMpeg site here
• Alternatively if you don't wanna look for all builds, the best can be downloaded from here

Launch System properties

• Right click on My Computer
• Choose properties

(or)

• Open run command using windows hotkey win+R
• Type sysdm.cpl
• Click ok

Launch Environment Variables

• Switch to tab named Advanced
• Click on button Environment Variables

Add ffmpeg to system's environment

• Look for the variable named Path in System Variables section
• Click on Edit button

• Go to the folder where you've placed ffmpeg
• Search for ffmpeg.exe file and copy the location
• Paste the location in the text box for Variable value and type ;
  leave the existing paths as is
• Click ok
  

Check if configuration is successful

• Open command prompt.
• Type ffmpeg, press Enter
• Ffmpeg information should displayed (if not something would have went wrong, try the configuration again)