Depends on the program applied for. Mostly determined by choice & funding of professors. Though the first year of funding is provided through a fellowship, a department can only hire number of students that can be afforded by professors on the later stage. So it's better to contact professors first.

Meet with the PI and go through three rotations lab. Normally each student will stay in each lab for a month. She would learn basics all the projects and the science behind them. Then if she likes the lab, she would ask to stay and wait for the fellowships