To take a pre-existing object or photograph which has a background you want to remove -use the extract filter in photoshop.
The extract filter takes quite a bit of experimentation.
You are going to have to read the manual on how to use this
But i took this image of some flowers and made the blue sky transparent.
You have to hand draw around the edges to help the filter, the 'smart hilighting' check box helps enormously. i used a big brush size to outline the edges. your brush size will depend on the resolution of the images.
the nicest thing about extract is that you dont have to be too accurate! just broad brush strokes, in close to the right place.