Can you be more specific about what you are trying to do? There are several ways to create transparency in Photoshop- the eraser tool, masks, selections, alpha channels. And there are probably about as many different tutorials available on the Internet. Have you searched Google for "photoshop tutorial cut out transparency"? (That's the first search string I would try.)
what i need is this: i have a picture for example with black background color. i want to add a button somewhere in the pic. when i do this.....the button is added but it also covers an unnecessary part of the black background making it look unnatural. i've heard that to make the button appear as a part of the original image, i have to make add button image as a transparency. but i wanna know how i can do that!
It sounds like you're working with multiple layers. If that's the case, use one of the selection tools (the rectangular marquee or the lasso, for example) to make a selection around the button, or around the area of the button that you want to show through to the background. Then choose Select menu > Inverse, and then press the delete key.