Step one:
Open an image with a dark background. Add a new layer and chose the eliptical tool. Now draw an oval big enough to go around your image, or where you want the effect.

Step two:
Next go to the selection menu/feather at 30 pixels. Keep image selected. Press D to be sure default colors are selected, then X to make foreground white.

Step three:
Now go to the Edit Menu and stroke at 50% from center at about 12 pixels. Keep image selected. This will still be very light.

Step four:
Then go to the Edit Menu again and stroke from center at 3 pixels. at 75% Still keeping the selection. Play with various sizes and colors to get the effect you are after.

Step five:
Next press Command T on the keyboard to transform your selection. You can rotate your halo to just how you want and/or resize and move it. Hit return. Now you can deselect. If it's not where you want it, press the letter V for the move tool and move your halo where you want it.

Step six:
We aren't done yet. Select your eraser tool with a big enough brush to cover the edge of your circle. Double click to bring up the options to be sure you have 100%, then you can erase the part of the halo on top of your image, leaving it to look like it goes totally around. Only covering half of your image with the ring. Now step back and enjoy your artwork.Enjoy playing with variations, I have pulled gradients through it also change the color to make halos over people. ;>)