I included this as an answer to another question, but felt it was relevant here. At age 17, I was working backstage at an outdoor music concert, and was asked to walk out on stage in front of 15,000 people to give water bottles to all the performers. I was terrified. I asked my boss a barrage of questions:

"How should I do it? Do I just put it in front of them? Or hand it to them? Do I walk behind the speakers or in front?"

My boss looked at me and said:

"Just go out there and pretend like you know what you're doing."

I've used that sentence as a mantra throughout my life whenever I'm doing something for the first time, and it's helped me immensely. And over time I've realized - everybody's faking it. So now I just pretend like I know what I'm doing, and before I know it, I actually do.