Changing Window size?

I just got the 21.5" iMac and to be truthful it is intimadating. I have learned a few things about it but I have always been a pc user and that was limited. Ok my question is whenever I open up a window or application as Apple calls it some of them such as my mail page is full screen and others are half screen, whenever I try to make the mail page smaller with the + and - button nothing happens can someone help me with this? Thanks in advance.

There are two sizes for any given window set by the green dot. Default and that which you set it to by dragging the window by the lower left corner out or in.
So with the Mail window, if you hit the green dot, it should go to a smaller window, and again and it should go full screen.

Lets say you drag in the lower left corner of your mail window 3/4 of the way in, and then click the green dot, it should go back to it's small size, and click it again, and it should go to that 3/4 position you just did.

When you drag any window to a certain position, each time you open that app, it should open in the last size you made it before closing or quitting.

Try it with the finder window now for a quick understand of the green dot, and the opening and closing of the window.

Thanks I think I got that figured out. How do you get the HD icon to appear on your desktop? Thanks

Go to finder in the menu bar, preferences, general, and select Hard Disks.