Welcome to the forums. I'm no expert, but I do have a week or 2 head start on ya. I'll start you with this---when you download an app from the marketplace, it install automatically. To find it, tap/swipe upwards on the little tab at the bottom of the screen (when holding vertically). All of your icons are in there. Find the icon for the app you just installed, and then press & hold it (called a "long press")... after a second, you'll get a little vibro-feedback, and the icon tray will disappear, and you can drag the icon to where you want it on any of the 3 home screens. Same goes for rearranging what's already on the home screen- long press, vibrate, drag & drop.
As for adding more screens, there are home page mod apps that add additional screens, like Home++ or Open Home, among others. You may want to read up on them in the Applications forum on the site.
Enjoy!