The English word you are looking for is 'godfather'.
Yes, the godfather must be a member of the Orthodox Church. Because it his responsibility to help you learn how to be a good Christian.
Normally, an adult chooses their own godfather (when infants are baptized, the birth parents choose the god parent). When you begin going to a church, you will get to know the people there. You find one who you respect, who you think will be a good example to you, and then you ask them to be your godfather.
Edit: My apologies, I just realized from the other thread that you are a woman. Normally, your godparent is the same gender as yourself. So you would be looking for a godmother rather than a godfather.