You need to remove the passenger seat to get things rolling. You can start here:
http://mike.focke.googlepages.com/replacingyouralternator
There's lots of discussions in this forum---use the "search". Here's a couple to get you started:
http://986forum.com/forums/boxster-general-discussions/19238-follow-up-alternator-questions.html#post174859
stuck in an alternator swap
If you're going to do much work on your Box, you should consider getting a shop manual!