First, get your image, im going to use this one:



Step 1:
Create a new layer

Step 2:
Using the rectangular marquee tool, make a selection roughly the size of the light rays you want. Dont be too accurate, as we have to transform it a bit anyway.

Step 3:
Fill your selection with black

Step 4:
Press D on your keyboard to reset your colours to default (or set your forecolor to black and backcolor to white)
Filters->Render->Fibers
Variance: 16
Strength: 4

Step 5:
Ctrl + T
Then rotate it so it is how you want it, your light rays will go the way this is tilted

Step 6:
Filters->Blur->Motion Blur
Angle: Depending on how you tilted your light rays, this will vary, so look at your light rays and set it to *roughly* the same direction.
Strength: 355

Step 7:
Set the blending mode of this layer to Soft Light

Step 8:
All Done

My Result (with a few more effects applied to make it look more "magical") :



Enjoy

its diferent

I would suggest using a image with a much higher quality for a tutorial like this, the starting image looks horrid, and the finished product looks worse. Not because of what you have done, but because of the crap quality.