First off make a gradient going black to white. It doest matter which way its goes.
Step 2: Difference clouds
Go to Filters> Render> Difference clouds.
Step 3: Invert!
Easy enough, click control+i, or go to image> adjust> invert.
Step 4: Levels
It's kinda looking like lightning already! Now, go to image> adjust> levels (or control+l). Move the middle slider to the right. The farther left it is, the less the clouds reflext. The farther right, the more they reflect.
Step 5: Color!
Make a new layer above the lightning layer. Fill it in with a light blue (alt+backspace). Set the layer mode to overlay (in the layers pallette, right above the little thumbnails).
Step 6: Tada!
Your done your lightning. Play safe and don't electrocute anyone! :D