What the others have said: Be honest if you don't know the answer. Say "you know what? I don't know the answer to this, but I'll bet Google does!". There are literally millions of error messages, you can't be expected to know them all. Just treat it as part of the process, not as a failure. As long as the customer sees you work through the problem, they never complain that you didn't know the answer right away.
I remember my first call like it was yesterday. Unfortunately the appointment was made by the actual customer's son, who wasn't there when I showed up. The woman spoke spanish with only a very rudimentary and broken capability for english. What I SHOULD have done was apologize, reschedule when her translator could be there and leave.
Unfortunately, what I actually did was to struggle with her computer for an hour, replace a NIC that I probably didn't need to, all with her hovering above me and convinced that I was ruining her computer. She did pay me after a 20 minute debate (including a phone call to her son with a heated conversation in spanish), but I was so anxious to get out of there that I didn't realize until I got home that I completely forgot to charge for the part.
Today, this would probably be a 10-minute job, but looking back, I'm surprised I didn't just pack it in after that experience. She never called me back, btw.
I've done about 10,000 calls since then, and I can count the "unfortunate" ones on one hand. They make good war stories, though.