If you're joining a pre-established team, you'll likely start off dealing with printer toner, IWB's not working and network connectivity. They'll most likely have a system for common problems already in-place, just work on working your way into it and 'getting with the programme'. As a techie that recently completed an apprentice, I can give you the following advice:
All of my laptops can't login! = Billy forgot his password, 2 laptops have the wireless switch set to 'Disabled', and the rest of them are fine.
My computer won't switch on! = Monitor is turned off.
The internet is down! = One website is being slow.
There's no sound! = Volume is muted or headphones are plugged into the microphone port.
Billy's files have gone missing! = Billy deleted his coursework again.
It won't print! = Toner is empty.
Also be prepared for many, many vandalised keyboards (I had one just yesterday where the top row of a laptop had been changed to 'YOURADICK'. Props for creativity, minus points for spelling. C'mon, the E wasn't even in use!) and vandalised mice. Some of our kids like to bend the mouse buttons back and snap the.. 'legs?' underneath them so they don't press down on the button inside. That happens a lot. Forgotten passwords and files going 'missing' are also fairly common but easily resolved provided backups/shadows are enabled.
There are, many 'genuine' problems, granted. We have a few sets of laptops that randomly like to disable their own wireless, so you have to log on as local admin and func+F5 them back into life, we have an ongoing problem with one room failing to get its printers (even though the policy is identical to another room!), laptops get dropped and the hard disks fail so we need to order new ones and such, but realistically your common problems are 90% user error. You'll learn what your staff continuously get wrong pretty quickly.