I know Axver was looking into sending some people PT albums, but here's a blog where you can get all of them if you wish (I'm assuming the links work - note that the password is bunalti.com).
http://bigrock666.blogspot.com/2008/04/full-discography-porcupine-tree.html
Now this discography lists everything, included compilations, singles, EPs, early demos, etc. For now, if you're new to PT, stick to the regular albums, which are:
On The Sunday Of Life
Up The Downstair
The Sky Moves Sideways
Signify
Stupid Dream
Lightbulb Sun
Recordings
In Absentia
Deadwing
Fear Of A Blank Planet
I've separated them into three groups, as PT have gone through three phases in their career.
The first group is more ambient and experimental, and at times sounds like Pink Floyd (see The Sky Moves Sideways).
The second group is probably their most accessible, as they streamlined their songs for a more rock sound.
The third group goes a bit further, incorporating progressive metal into their music.
So choose the group that you think you'd find most interesting first.
My pick for the first group would be The Sky Moves Sideways, for the second it's Stupid Dream, and for the third, well, take your pick. Any one of them would work.