File > New
Choose 2 colours that you want to use (if making a brush just use Black & White)
Click the Gradient Tool (G) > Set to Foreground to Background>Linear Gradient>Mode: Normal>Opacity:100%
Click and Hold mouse at the top of your canvas, Hold Shift and drag mouse to the bottom of your canvas. This should fill your canvas with a gradient.
Then
Filter>Distort>Wave - Set so you have equal lines running vertical, may need to play with the settings here depending on the resolution you are working with. I normally set 'Generators' to 299 though. Experiment with the other settings.
Once you have completed the above step you should have a canvas with alternating lines vertically.
Click
Filter>Distort>Polar Coordinates and again have a quick play.
Click Okay and you should have rays.
Here's a quick video I made for you too: