How does one become a cox?

I am not so familiar with the terms for each players of rowing. I just know about the cox. Do you need to acquire a certain skill to become a cox or it is based on the seniority in the team?
The coxswain needs to be a good communicator, he/she will direct the rowers and steer the boat. The lighter the cox the better, the rowers will have less weight to move. Coxwains usually weight around 100lbs.