I asked the same question when I first got mine. You can go to settings and force stop programs, or just leave them open.
The other option is to keep hitting back until you get to the desktop effectively closing the program.
The best bet is just leave it alone. Android is way smarter about memory management than iPhone or you, the user.
Apps not in the foreground don't use any significant memory, and an immeasurably tiny amount of CPU.
When you move away from an app, a small bit of state information is saved, the rest of the app is kicked out of memory.
If you go back to that app, it will use the state info to re-establish the app as it was when you left it.
Those apps that do run in the background (like your mail programs) you want to run.
There are a very few optional apps that once you start they continue to run in the background and use CPU/Memory.
(Music players, nav apps, etc.) These always have an exit button, or become in-active when you stop playing music or navigating or what ever the app does).
Unless you have a problem app, that just seems to suck your battery dry, leave it alone. If you do find a battery-sucker, delete the app and go find a better one.