The rules are going to change depending on whether you have a firewall which all traffic passes through, or the firewall is on the actual machine you want to protect. I've written the rules assuming the latter.
But if you have a dedicated firewall box which protects other servers, you'd probably want
-A FORWARD
Instead of INPUT our OUTPUT.
The easiest thing to do would be to allow all outbound traffic for that server. Or you could be more specific, its up to you.
# Allow outbound traffic from MyServer
iptables -A OUTPUT -j ACCEPT -s $myServer
Where $myServer is whatever IP you have. Not sure about how you'd do this in webmin, sorry.
Whether you want to allow all outbound traffic, or be more specific, is up to you.
EDIT:
You may also find it useful to allow inbound ICMP (for pings), if you haven't already.
iptables -A INPUT -j ACCEPT -p ICMP -d $myServer