Attached Link: Layermask on myjanee.com

Wow. I guess one first resource for your question would be the Photoshop manual... Because a mask is just what you are asking, something you can use after making a new selection. And, even better, all the pixels are still there if you decide the antenna has to be on the roof of the car. The simplest resource might be trial and error (it worked for me). Open up a file of a car, or something. Double-click in the layer tab on the layer. It will become Layer 0. Click on the mask icon (circle in square). Now you have two rectangles in the tab. The original photo and an empty square. Pick a tool (take the brush... to keep it simple), and paint with black in the empty square. Make sure it is selected, you will see that with help of a line around the little thumbnail of your layer. As you will see, you can paint away the background of the car. But what if you accidently paint over one of the mirrors and find out a long time later? No problemo! Just erase the parts where you crossed the lines! Masks are a gift from God! A lot of people do not know this, but you can actually look at your mask at full size by alt-clicking on them (it might be some other key-stroke on pc, but you will have to click). You can use filters on masks, use airbrushes, clone stamps, healing brushes, whatever you want. Without changing the pixels in the original picture! If you decide NOT to remove the background after all, just throw away the mask, and it pops right back! You can read the discovery of heaven (which is a book by a Dutch writer, but not what I mean) in the link I provided... I am quit sure you will like it a lot. A whole new world of possibilities will open up, one of the greatest things is that you can temporarily 'erase' stuff, and you do not have to start all over again if you change your mind. That's the computer age in full effect! Good luck pioneering!