How do you specialize in nursing?

I am in the process of getting accepted into nursing school. They might accept me for the spring semester but they guaranteed me for the fall 07 semester start as long as I pass the rest (3) general couses I still have to finish which I will be doing this fall. My question is, how do you specialize in nursing? I would really like to specialize in Psychiatric or neurology and can't see myself working for anything else, this is what I want to go to nursing school for. How do I make it happen?