Well..,since you have a stock iPhone, you will have double press the home button, tap and hold on an icon. Then tap the minus sign. Them the app is quitted.
And FYI:
Apps do NOT run in the background. Certain apps can, like streaming apps, but like 99% of all apps do not run in the background. They are paused in RAM.
There is no way the apps you leave open are draining battery and using data, while you are not physically using the application.
Since I'm Jailbroken,,....I just tap and hold on my status bar, and all apps quit at one time. Removing them from the switcher.