I’ll answer this one in two parts.
First, nerves are a sign that you care. If you weren’t a little nervous, I’d wonder why you’re even playing the game. Calm the nerves by doing something you’ve done thousands of times before – your pre-shot routine. If you don’t have one, DEVELOP one! If you’ve heard “focus on the process,” that’s what this gives you: something to focus on that you’ve done thousands of times before.
Second, and this isn’t for everyone or in all situations… tell yourself that in the grand scheme of things, whether you shoot the best or worst score you could possibly shoot that day, your life probably won’t change all that much. You’ll still have your family, your friends, your job… You’ll feel good or bad for a few days, but overall, the sun still comes up in the east tomorrow.