Building a lasertag gun is easy, building a good lasertag gun is slightly harder but well within the abilities of anybody.

All of them have several things in common though, circuitry, power supply and lens unit, they also usually have a speaker for a sound effect and quite commonly a muzzle flash that lights up when the gun fires (to comply with UK games rules a tag gun MUST have both sound and muzzle flash).

To be effective it doesn't have to look good but its always nice when your friends go 'cool'.

While it doesn't HAVE to look good, one thing that you really must think about is ergonomics. Remember you will be using it, possibly for several hours at a time so its got to be comfortable to use. Study how real guns are designed, the relationship between the different parts, where displays/selector switches will be located etc.