Here's how to take screen shot on any mobile operating system if you can't find a Take Screenshot button on the screen or don't want to install any kind of app to do the same.
Android phone (Android 4.0+):
Method: Power button + volume down button (or) Power button + Home Button
Screenshot location: Mostly camera shots, you can find them in Gallery.
Method: Power button + Home button
Screenshot location: Camera roll
Method: Power button + Windows key
Screenshot location: Screenshots album in Gallery
If they don't work, comment, I will help you out. More operating systems? Suggest me, you know the comment section is already a lonely place...