Riotur (Rio's Tourism Department) coordinates the Carnaval

http://www.riodejaneiro-turismo.com.br/en/home.php

More info the the Samba Schools League site

http://liesa.globo.com/

For now, all they have is the ticket price, no indication on where or how to buy.

My advice as a Brazilian: unless you are really, really keen to seing the Samba in Rio thing, consider going to SSA (Salvador, Bahia) instead. It's way more fun, better music, better location, more "democratic" (as in street Carnaval versus Rio's closed venue scheme)

Maria