There are two type of lasers:
a) Continues Ray
b) Short laser pulses

Continues Ray (Easy way)
 

1. Create a Taget Directional light. Change the color to the one you want (eg orage). Change the multiplier to 5. Change the dimesion: Falloff: 8, Hotspot: 4 . Enter the following shadow settings: Shadow map, bias: 1, size: 32, smp: 1 .
2. Go to the Render menu and choose environment. In atmosphere add a Volum Light. Leave the setting, only add the light to source.
3. Now it should look like this. If you want to animate it, go in the Trackview and animate the multiplier track.   I will write a tutorial about creating realistic lasers soon.

Short Laser Pulses
 

1. Create particle system. Change the particle count to 12-20. The speed to 150.The emitter sould be very small. The life is depending how far the target will be.
2. Go to the objects properties. Activate the object motion blur. Change the G-Buffer.
3. Go to the Render options and change the motion blur option to this.
4. Now it should look like this.
5. Go to the video post. Add a scene event and the super glow filter. Change the options to those on the picture.
6. Render it. Now it should look like this.