Open a browser and enter the following URL.
Login using your user id. You can also login using the administration user id you set up in Ubuntu, if you created one. Now you are in Webmin looking at a status summary on the right and a huge selection menu on the left.
If you know the IP address of your Webmin box on your local network, you can login from other computers on the network. My Webmin box has an IP address of 192.168.0.145 and I can access it using https://192.168.0.145:10000/. If your Webmin machine does not have a fixed IP address, login to your router and allocate a fixed address.