You're going to need a basic familiarity with the software. Play around in it.
Try creating your own custom banners for webpages you like. I used to do that as little "fun" practice lessons.
RGB stands for full colors. That means you can mix red/green/blue to get any combination they allow. I think it has millions of colors, or hundreds of thousands. I don't remember. It's essentially 16 bit (32 bit if you use alpha channelsl!)
Indexed mode (another term you will learn while skinning) is limited to 8-bits or less, this means 256 colors or less. It's a restrictive format compared to RGB, but you can still pull of some nice things.
There is usually an image menu with something like "mode" as a submenu, and in this mode you will find RGB, indexed, greyscale, etc.