How do I hide apps on the desktop iPad?

Some can be hidden via Settings > General > Restrictions (YouTube, Safari, iTunes, Camera, FaceTime and App Store via Installing Apps). Others can't be hidden (e.g. GameCentre), but you could put them in their own app folder and put that on your last homescreen so that they are less visible.

you drag an icon on top of an icon to make a folder

about excel then you most likely need a 3th party program to view and edit excel sheets

look in app store

If you press and hold one of the icons on your homescreen, then after a couple of seconds or so they should start to shake. You can then delete apps via the 'x' in their top left corners (though you can't delete the built-in Apple apps), or drag them around the screen so as to re-arrange theie order. If you drag one and hold it over another then a folder should get created with those two apps in - you can then drag more apps, up to 20, into each folder. Press the home button when you've finished deleting/rearranging so as to stop the shaking. You can also do it when connected to your computer's iTunes via the device's Apps tab (which some people find easier to do).

There is a bit more info about doing it directly on the iPad in the manual whch, if you havn't got a copy, can be downloaded from here : http://support.apple.com/manuals/#ipad