To close apps, double click the Home button (or do a 4-finger swipe up if you are on iOS 5) to reveal a bottom row of icons. These are all the apps you've had open. Press and hold an icon until they all start to wiggle and you see a red circle/minus sign in the upper left of each icon. Press the minus sign ("-") and the app will disappear from that row. This closes the app out entirely - though do note (or don't worry), you are not deleting the app from the iPad.
There is much discussion on how many times this action needs to be done. Have a read around the forum and form your own consensus. Hopefully, whatever you decide, this will help you "clean up" your iPad.
Marilyn