Just got hired as a computer tech

I've been working with computers for quite some time now, but I've never had a job doing it professionally. I've built a couple computers and done basic maintenance, networking, malware removal, etc. Recently I decided to get into the computer repair field while in college so I can get out of working in food service.

I was just hired at my university's computer service center as a tier one repair tech. The job responsibilities mainly revolve around virus removal, installing/configuring software, possibly some hardware upgrades, fixing network problems, etc. This stuff is all easy, but I wanted to know if anyone has some basic advice on how to efficiently fix someones computer. I guess I need a method on how to approach problems so I'm not wasting time or trying to fix problems that aren't relevant to the customer's problem.

I guess I'm just looking for some overall advice from people that have been doing this for a while. It will help my confidence level when I start the job, as nervousness tends to impede one's knowledge.

The job also entails working on Macs a lot (many students use macbooks). I've never really worked with Mac, but I'm sure I could pick it up quickly.

Thanks for any advice. I'm so happy to be getting some experience with IT instead of serving people Lattes at Starbucks.