I am building my first sub also, I am just waiting for my warrenty replacement shiva to arive and I am done. (the first was defective, very fast replacement. good company)
I lurked and searched on this website for awhile then posted some questions.
Almost every question I had was already asked so I would do some research. If you do not feel like doing that I would build one of adire plans w/o mods. people swear by them.
with that said, I would download unibox404. it is a shareware excel program to model speakers.
I knew nothing when i started so I searched and found other peoples suggestions and worked backwards to try and get the same answers. this taught me the software.
Then just start tweeking. You will need to figure out volume of the cabinet you want and figure out the port size and length.
when done you can post your finding and ask someone to check them for you.
here is another website that has good info on differnt types of subs.
http://www.diysubwoofers.org/
good luck and have fun.