i think this questions gets asked at least once a week.
check out the debian cd FAQ
http://www.debian.org/CD/faq/#which-cd
or if you are too lazy for that. basically the net install cd is just to start up
your system to installation, the rest of the packages get downloaded
during installation. i would recommend the first cd because you're going to
use most of those packages and if you stuff something up you don't need
to redownload alot. Any other packages that aren't on the first one you
can just download when you use apt-get.
but basically you need only the first cd to install debian, the rest are
just extra packages that you can download as you need them, becuase
most you wont use