Your question is a little vague. Understandable if you're new to this.
I think what you're trying to say is "How can I make portions of this image clear and keep others the way they are?" Right?
You need to select the areas you want to knock out then mask or delete them. I prefer the former whenever possible.
Let's say you want the button shape pulled off of the dark grey background.
One solid option is the Rounded Rectangle Tool. Use it to draw a shape around your button. Notice my guessed Radius value for the corners. Adjust as necessary.

When you have your path where you want it, use this option in the Layer menu and you'll end up with a handy dandy vector mask. It shows up in your layers panel as shown below.

Now when you use File > Save for Web & Devices ... you can save to a PNG or gif file and check transparency to get something like this. (Forgive my inaccurate path, busy morning ;)
