Nikon Sb-900 Nikon D90?

So I just bought the SB-900 and mounted on my D90 but for some reason when I take pictures the SB-900 won't flash what should I do for the wireless setup thing?

For on-camera use, simply set the SB900 to ON.

For off-camera use, set the SB900 to Remote, and configure your D90:
1. Press the Menu button on the back of your camera.
2. Go to Custom Setting Menu option (pencil icon).
3. Select e, Bracketing/flash, menu and press OK.
4. Select e2, Flash cntrl for built-in flash, and press OK.
5. Choose Commander Mode.
http://www.umeshupreti.com/camera-controls-and-settings/nikon-flash-and-commander-mode/

The SB900 will be triggered by the pop-up flash, so the two require a line of sight. Otherwise invest in radio triggers such as pocket wizards.

When you use the pop-up flash to trigger the SB900, you have the option to either include the light from the pop-up in the image, or to use it only as a trigger. It's explained in the link.