tuning a ported box depends on the size of the box, and the size of the volume. typically, given the square inch size of the port, making it longer will lower the tuning. most boxes around here are tuned to low to mid 30's. the bigger the box...the bigger the port.
EDIT: the formula can look weird and intimidating. but if you know what all the variables stand for, then there's nothin much to it but basic math. there's lots of people to help here, and a thread dedicated to port tuning and such. what subs you plan on running?
EDIT EDIT: and welcome to the site btw