Well, there are lots of things to do. It really depends on the size of the organization you're working for. If you're working for a small company, you'll probably end up doing everything. If you work for a larger corporation, then things will be split into groups and you'll have a more specific role (installs, DNS, LDAP, NIS, troubleshooting, whatever). I'd recommend you try to get some certification if you aren't proficient with the command line, I don't know any sysadmins who use webmin. Hope this helps!