Get an image: http://www.raspberrypi.org/downloads
Put it on your SD card. Good instructions for OSX or Unix: http://ryanphilipsullivan.blogspot.com/2012/06/raspberry-pi-getting-started-with.html
Run the config program:
sudo raspi-config
Choose Expand root partition to fill SD card. Otherwise you will probably only have 4GB of usable space.
If you want a headless server (ssh only), you can change Start desktop on boot? to 'No'.
You need to reboot to see these changes:
sudo reboot
Check that you're using your whole card with:
df -h
You should see at least one line in the ballpark range of what your card advertises. My 32GB card produces a Size of 29G.
Install some packages:
# update sources
sudo aptitude update
# upgrade all packages
sudo aptitude dist-upgrade
# util
sudo aptitude install -y zip
sudo aptitude install -y rsync
# dev
sudo aptitude install -y apache2
sudo aptitude install -y git
sudo aptitude install -y nodejs
sudo ln -s /usr/bin/nodejs /usr/bin/node
sudo aptitude install -y npm