We will start off with a blank, black canvas and will use an image of a lightsaber handle:

Next, use the Polygonal Lasso tool, or the Rectangular Marquee tool to make a selection that will be the blade, and fill it with White (in a new layer). You can round the edges with a Brush or Select > Modify > Smooth:

Finally, open up the Layer Styles for the blade layer and add an Inner Glow and an Outer Glow. Choose your color and make the Spread and Size of the Outer Glow pretty big (20+), and the Choke of the Inner Glow should be 0, while the Size should be around 12px. Play around with the values to get the best result, as it will depend on your image and dimensions, etc.

As an optional, subtle effect, you can add a gradient or brush the top of the lightsaber handle to create a more atmospheric feel, because the light is so intense, it is bound to reflect off the handle. We add the reflection into its own layer, change the Blending Mode of this layer to Color, and reduced the opacity to 50%. It's a subtle change, but sometimes those are the best: