Although iOS is supposed to manage resources automatically, sometimes an app can hog memory or CPU resources and make your iPhone sluggish. Here is how to kill an app.
1) Double tap the home button (round indent with the little square at bottom of phone). This will show 4 of your running apps.
2) Press and hold any of the icons and they will all start jiggling with a red minus sign (-) in the top left corner.
3) Tap the minus sign (-) to kill the app. Swipe left and right to see other apps you can kill.
4) Press the home button when you are finished killing apps.