Start a new document with size 500px by 500px, and fill the background with black.

Create a new layer.

Press "D" on your keyboard to reset your foreground and background to black and white:

Go to Filter->Render->Clouds

Try to get clouds that are evenly spread between grey and white and not too much black. the image to the left is good.

Then go to Filter->Texture->Patchwork

Square Size: 10
Relief: 7 (you can play with this setting)

Select the circular marquee tool , then while holding down the shift key, click and drag to create a circular selection. Make it fairly big. (Holding down the shift key allows you to create a perfect circle selection)

Then go to Filter->Distort->Spherize

Amount: 100%
Mode: Normal

Now we got the disco ball, we need to get rid of the surrounding.

Go to Select->Inverse

Then hit Delete. This will remove the surrounding.

Select the Brush tool and in the brush menu, scroll down and select either one of these brushes:

Set your foreground color to white, and make a few shines around your disco ball.

Because the ball is on a layer of its own, you can use it on any color background.

Enjoy!