You will need 2 pieces of information:
The name of the external interface (eth0, wlan0, ppp0 etc) and the public IP address of this interface.
# Enable IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

# Enable SNAT (Source Network Address Translation)
# so that all PC's on the internal interface will be NAT'd to the IP of the public interface
iptables -t nat -A POSTROUTING -o EXTERNAL_INTERFACE -j snat --to-source EXTERNAL_IP

Here is an example configuration for 2 wired network cards eth0 and eth1
auto lo
iface inet loopback

# The external interface
auto eth0
iface eth0 inet static

# The internal interface
auto eth1
iface eth1 inet static

Enable IP forwarding and SNAT
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o eth0 -j snat --to-source

Now set the gateway of all machines connected to your internal interface to (address of eth1).
Remember to set up a firewall on your external interface!
You can do this with iptables (the hard way) or using a Debian firewall package such as shorewall, firehol, or ufw.