Welcome to iMore! You can force close an app by double pressing the home button to open App Switcher and then swiping upward on whatever app(s) you want to close. That being said, iOS does a fine job of managing system resources for you so you don't really have to close apps out yourself. An argument can also be made that closing out apps only worsens battery life because it takes more resources to open them back up when you need them.
I recommend letting iOS handle your resources for you.