You can find tons of information at many places. One place to start is probably at Apple's
Nano support page
You can't do YouTube like you can on, for example, the iPod Touch or iPhone but you can download videos.
If you're totally new to using an iPod, then the first thing to get used to is the way it's designed it to integrate with iTunes on your computer. It's not really a totally independent device. If you kinda think of it as an accessory for iTunes instead of the other way around then maybe things will be more clear about its capabilities. (The iPod Touch kinda finally starts coming into its own finally, but the Nano is still a dongle for iTunes in my opinion.)
So think of iTunes as home base. Then ... if you can get it on iTunes you can afterward sync it into the Nano and take it with you.
I hope this helps somewhat.
Oh -- yes you can use 2 on the same computer but unfortunately you have to build 2 separate libraries, one for each iPod. You'd be best off if you just create 2 separate accounts on the computer (mac or pc) for many other reasons and it also simplifies using iTunes too, though it's not too bad on the same account.