A first day is just that -- like kindergarten. Just let the fear pass through you. It's real, it's a feeling, and all feelings pass. For me, that was the hardest part of getting sober, the feelings, actually feeling them. They sucked for a long time. Many still do.
But at least we aren't numbing feelings to nothingness now. You are supposed to be anxious, and humans (those normal, non-addicted types at your new job) will allow you to be nervous. Just kill them with kindness, humility, and something that is hard for this alcoholic and addict: Honesty. Just smile and say, "Hi, I'm nervous!"
Gets 'em every time.
Let us know how you did. Good luck. Go with the flow. And come home from work tired, a little frustrated from the mistakes you make, the social snafus you might tumble into, and relax. Take a shower. Watch some TV. Get your clothes ready for the next day of work. Take your meds. And chill. And feel proud.
And don't drink. Congratulations. You just knocked another 24 off the clock sober.