You'll buy a kit from a Nissan dealer that includes a tuner, antenna, and
instructions specific to the Maxima.  Go to www.maxima.org , you'll find
everything you need there including dealers who have the Sirius or XM kit
for pretty cheap online.

Your car's headunit receives the signal from the tuner box.  The antenna
connects to the tuner box, which you will install somewhere in the trunk
(there is an area for it).  Nissan, along with everyone else, had to break
the sat tuner box out from the head unit because a) heat, and b) to give
people a choice between XM and Sirius.

I personally have Sirius because the music is more to my taste (XM is more
eclectic, if you like that sort of thing), and they have the NFL.  I find
myself listening to the NFL channel (pro football talk 24/7) more than I
thought I would.  Sirius will also have Stern, which interests me none, but
you may care.  XM has Opie and Anthony, MLB, and NASCAR as exclusives.
There are other differences, go to www.siriusbackstage.com and www.xm411.com 
and www.xmfan.com for more info.  Both www.sirius.com and www.xm.com let you
sample their channels online (music only).

You really can't go wrong with either service, I've only bought 2 CDs all
year since getting my Sirius tuner back in January.