tuning a ported box

how do you tune a port in a ported box I've always heard about people tuning their boxes but I don't know what they mean?

how do you know what frequency to use

If you're trying to get the best sound quality, go through the learning process as mentioned above.  If you're not sure, it's safe to go with 40 Hz in most cases.  If your sub amp has a subsonic filter, use it somewhere below the tuning frequency...or if you don't have one, pay careful attention to the sub cone during the setup and make sure it doesn't flap out of control during the lowest sub freqs.  Hint:  commercials on TV that show subs almost always show them flapping out of control (for the visual effect of the commercial).  Those subs are ruined by doing that.Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.