How can I turn my iMac into an wireless (airport) base station.

apt-get install dhcp3-server
apt-get install bind9
apt-get install ipmasq

You may need to configure 'bind'; you definitely have to configure dhcp3-server.

Otherwise, the only other thing you have to do when you configure the wireless is to set it up as Peer Access and set the ESSID and passphrase.

With 'ipmasq' you can edit the configuration files later; the default values usually work, but aren't necessarily the best choices.

If you don't configure each bit of software as you install them (above), you can invoke dpkg to reconfigure them later rather than just editing the config files:

dpkg-reconfigure dhcp3-server

Of course the post-install scripts are not magic - sometimes there is no substitute for reading the manual and editing the config file. If you need documentation for 'bind':

apt-get install bind9-doc

You can actually do without 'bind' if you configure the DHCP server correctly. I usually install it because:
a. It saves me a little time setting up - all peers think my one machine is both the gateway and DNS.
b. some defective dhcp implementations don't accept the DNS address handed over by the server - they assume the DHCP server is also the DNS machine.