How to make this particles in after effects -- example included

this should get you started. just follow these parameters in CC Particle systems II effect. for the opacity I used an expression to flicker it back and forth (alt+click on the stopwatch and type the wiggle expression). now you can duplicate this layer and change the random seed parameter so you get a different flickering behavior for another instance of particles. make sure you move your solid layer far enough back so the particle get enough time to accumulate.

if all of this is too complicated for you, I would suggest you look for particle world/particle systems II tutorials. they follow the same principals - you adjust parameters according to physics adjustments and set them loose.