First of all
You should not use Webmin. The reason is that webmin easily breaks your system and do weird actions for simple actions. Plus it is not secure for online use.
You have ssh witch is quite enough (I assume that you said openssh server, if not install it)
As you can see you can enjoy a bunch of knowledge and people wishing to help : if you don't know how to do one thing google it, search here in askubuntu and if you can't find it ask a question here.
The Online issue
You need to configure internet on your server. For that you must specify the gateway and dns ips.
If your Ip is local :
open a console and type ipconfig. If you see something like 172.16.XXX.XXX or 10.XXX.XXX.XXX or 192.168.XXX.XXX then it's local (XXX being a number)
In that case you must access to your router and do some port redirection. THe port are the communications channels (TCP/22 for SSH, TCP/21 for FTP, TCP/80 for HTTP)
And then you must know your public IP witch you can obtain by searching "what is my ip" with a browser
If your ip is not local then you are online and the IP of your machine is the IP that you must access.