I'll tell you here what's to do to get up your network connection. Later I'll tell you how you can set this up permanently in `/etc/rc' etc., see section 3.5 How to use the above on `/etc/*'?.
Before configuring any network-device, let's first configure the loopback device:
# ifconfig lo0 inet 127.0.0.1
By now, you should be able to ping 127.0.0.1.