The reason that most people seem to be right handed is because being right handed is the dominant trait, over the recessive trait of left handed. That means that in order for someone to be left handed, they have to have two genomes(?) of the recessive trait, and none of the dominant, for the dominant will override the recessive. That's a one in four chance of having two of the recessive; so I guess approximately one in four people are lefties. :P
For example, in my family my mother and two brothers are all right handed. However, my dad is left handed, and so am I.