cant install npm discord.js

 npm : The term 'npm' is not recognized as the name of a cmdlet, function, scri
pt file, or operable program. Check the spelling of the name, or if a path was 
 included, verify that the path is correct and try again.
At line:1 char:1
+ npm install discord.js
+ ~~~
    + CategoryInfo          : ObjectNotFound: (npm:String) [], CommandNotFoun  
   dException
    + FullyQualifiedErrorId : CommandNotFoundException