If you're just doing this on the fly
ifconfig ce0 plumb (or qfe0, hme0, whatever your NIC device is)
ifconfig ce0 <ip_address> netmask <netmask> up
echo "gateway_ip" > /etc/defaultrouter
vi /etc/resolv.conf and add your DNS information
To make permanent, you'll need to add entries to /etc/hosts /etc/netmasks and create an /etc/hostname.ce0 (or qfe0, hme0, whatever) file with the hostname in it.
There are other things to do depending on what you're doing.
These tasks will be covered in basic system administration texts.
If you get stuck with a particular step, please don't hesitate to ask!
Cheers
ZB