I wrote this guide as simple as possible so don't be afraid if you're a total photoshop beginner. I also created the video version if you prefer to learn via video.
1) Open any photo that you like, in your bottom right corner (layers panel) click 'Create a new layer', we'll work on this layer from now on. (Leaving the original photo untouched)
2) Select 'Line Tool'
3) Set width to like 10px or more (depending on your photo you may need to increase the size, this will be the size of your saber) and draw a straight line.
4) Click 'Add a layer style' button and select 'Outer Glow'
5) Click 'Set color of glow' and select any color (I picked green.. feel free to choose the dark side instead :) )
6) Set 'Spread' to 12% and move the 'Size' slider left and right while observing your saber, click OK when it looks right (trust me, any starwars fan will know)
7) You can stop here if you're satisfied, though a minor tweak can make your saber more realistic, continue reading step 8..
8) Click 'Filter' -> 'Blur' -> 'Gaussian Blur'
9) Move the slider left and right while observing your saber until it looks right (trust me, you'll know)
10) Below shows before and after applying Gaussian Blur effect, do you see the difference?
11) May the force be with you, always.