Specifc to kids, by middle school, they should be ready for a full sized guitar even if it's slightly too big. They'll grow into it quickly. I played a full sized Fender Stratocaster starting in 6th grade, which was my first guitar. Prior to middle school, you might want to check out a Mini Stratocaster, which has a smaller body and neck, and will be easier for a growing kid to play. Here's a review:

http://www.youtube.com/watch?v=WVaZ_teDZzU

If you are a total beginner then there are "Packs" that you can buy that come with a basic guitar, practice amp, and cables for $250-$400.  I would look into what kind of guitar your favorite guitar players use and then look for a beginner model that matches.  The electric guitars sound is very personal so if you are drawn to one kind of sound (for example Led Zeppelin,) then you would want to get a Les Paul guitar.

Here is a quick video that talks about some basics of the electric guitar.

https://www.youtube.com/watch?v=_RvmZXPsGZo