- desktop (human!) support
- networking inside buildings
- networking between buildings
- server maintenance
- OS administration
- authentication systems
- local services (DNS and NTP are the most important, but your end users will think of file sharing and printing as most important)
- external services
- security (information, yes, but possibly physical systems, too)
- backups (nobody cares about backups, they only care about restores)
Figure out which of these are currently running, then which are your responsibility. Document everything, and save copies as automatically as possible - you have four sites? You want five copies. One in each location and one in a secure off-site location.
Make lists. Review them in the morning and before you go home.
Get a copy of Limoncelli and Hogan's book, The Practice of Systems Administration, and the follow-up Time Managment for Systems Administrators. Expense them.
Later, figure out costs.
Good luck.