You don't usually need to worry about closing apps. A correctly written app will be suspended when it's not being used, and removed from memory if the memory is needed for something else. Conversely if you go back to the app while it is still in memory it will restart faster and using less power than if you shut it down.
You can close an app using the applications menu. Some launcher apps include task managers, and recent android versions let you close apps when looking at the list of recently used apps. But to be honest unless you have a rogue app there is no real need to worry.