I don't know anything about webmin, but if you want to use LVM with the new drive you would probably want to create a new physical volume. This page shows how to do that: http://www.thegeekstuff.com/2010/08/how-to-create-lvm/
If you don't really care about LVM then you just need to run "fdisk /dev/sdb", "mkfs.ext3 /dev/sdb1", "mount /dev/sdb1 /path/to/mountpoint". Obviously you would use the actual names of your disks and partitions if it's something other than /dev/sdb, etc. This link has a more step-by-step breakdown: http://www.yolinux.com/TUTORIALS/LinuxTutorialAdditionalHardDrive.html
If you aren't sure if you should care about LVM or not, here is an example of how you might use it. Suppose you have your drive mounted as /home/yourname/mediafiles. And then you max out that drive by storing a ton of media. With LVM, you could just add another drive to the volume group, and you've got more space. The files in that directory might be stored across a bunch of physical disks, but with only one mount point. You don't have to know or care which specific disks each file is on. You just look in /home/yourname/mediafiles and all your media is there.
The downside of that (I think) is that if one of your drives crashes, the whole volume group can be compromised. So I wouldn't use the above scenario unless you had really good backups or you didn't care if you lost your media.
If you've only got one disk, and that's all you will ever have, there's probably no good reason to use LVM.