What are these Blood Elf and High Elf, what are the difference and how do i become one of them ?

They are characters from other games - You cannot be a Blood or High Elf in World of Warcraft.

Kel

I'll try to break it down easier. First off you cannot be anyone of those = there are only 4 races on each faction. In the first real war between the Night Elf and the Burning Legion, high elves existed. High Elves were night elves that practiced magic. Because they led the Burning Legion into Azeroth through the well of eternity, after the war they were exiled. They traveled far away to the new continents formed by the explotion of the well. They later became known as "Day Elves" and, like Moon Wells, you will see Sun Wells when you play Warcraft III. Basically, after Arthas dystoryed Quel'Thelas, they wanted revenge for their people and proclaimed themselves Blood Elves.

there will be two new races with the new expansion and thats blood elf for hord and for the alliance i dont know

in the new expansion you can be blood elf on the horde side

draeni apparently for the alliance...although there is much speculation

Blood elves and Highelves are the same thing, An you cant be one till the expansion comes out.