I'm a new to droid (I just got this today).
I downloaded some apps from Market Place and it says "Installed", but I don't know how to put them on the home screens.
either open the app drawer on the bottom of the screen and pick the app you want and long press the icon. the drawer will close and allows you to drag it wherever you want on your screen or hit the menu button and hit add - shortcut - application and choose the app you want.
The next thing you will probably want to do is delete some default icons or remove some you added that you decide you no longer want. To do so "long-press" on the icon until you feel the phone vibrate and the icon changes size. Then drag the icon to the icon at the middle of the bottom row and hold it there until it becomes a trashcan and turns red. Then you can drop it and the icon will be deleted. To move an icon to a different home screen long-press the icon and then slide and hold it against the left or right side of the screen until the screen scrolls and the icon is moved.
Long press on an open part of the screen you want the app/widget on and that will open a menu...app, widget, folder, or shortcut. Pick which you want and it will load on the screen