Here are the list of elements you can pick. You can only control one and one alone. Also some races have a restriction on what elements they can and can not use so please check that out.

Air
Darkness
Earth
Fire
Light
Metal
Water

Each person controls each element differently. While one person who uses water can make big waves another can just simply make water balls. This all depends on how experienced your character is. They can only use there element if they have a way to use it. Someone with fire can only use it if they have fire available to them, same as water and metal. Tough air, earth, light, and darkness are easier to obtain as they are all around. No element is stronger then other other, tough some are weaker such as fire to water. Tough it mostly depends on the user, and how strong there power is.