How do I close all the apps that are still open?
iPad
Go to the home screen first by tapping the home button. Double tap the home button and the task bar will appear with all of your recent/open apps displayed at the bottom. Tap and hold down on any app icon until it begins to wiggle. Tap the minus ➖ sign in the upper left corner of the app that you want to close. Tap the home button or anywhere above the task bar.
You don't need to.
The "task bar" that Demo mentions above is not, in fact, a task bar at all - it is the Recently Used Apps list. Not all of the apps in this list are open, running or in memory.
iOS automatically handles closing apps that are not in use, so you don't have to manually manage them.
Read this article from Apple for more information: http://support.apple.com/kb/HT4211
Pardon my using the incorrect term for the recents tray.
You can close the apps in the recents tray if it is your desire to do so - whether the iOS is supposed to handle memory issues for you or not. I have done so any number of times when using my iPad and it has seemed to help me in the past with certain issues. It certainly will not hurt.
Not every iPad works the way that the "book" says that it should work. At least in my experience it hasn't.
How do I close all the apps that are still open?
