You do not need to stop apps from running imt background because almost no apps actually do run in the background. That, however, is for a different discussion.
If you want to see your recent apps - double tap the home button to bring up the task bar at the bottom of the iPad. Tap and hold down on any icon in there until all of the icons wiggle. Tap the red minus signs to close the apps. You can swipe in the task bar just like you swipe from screen to screen - in order to get to all of the recent apps.
Tap the home button - or tap the screen above the task bar when you are done.
Read through these links to understand iOS multitasking...
Note that the vast majority of apps do not run in the background. What you see in the tray after double-clicking the Home button are recently used apps.