Because of all the excitement surrounding your first event, it's easy to start at a faster pace than you've been training for. But it's extremely important to start at a slower pace than you think you're capable of. The pre-race adrenaline will wear off sooner than you think.
Once you get about halfway through the race, you can push harder if your body feels good. Part of the fun during these events is testing your body's capabilities. Simply increase your pace to what you think is sustainable for the remainder of the course.
Don't feel performance pressure during the race. Many people simply walk their first 5K, and if that's all you can do, that's just fine. You can also do a combination of walking/running if that's what you can handle. Just make sure to go at a pace that's sustainable for you and that will still leave you smiling when you cross the finish line.
That's it! Now that you know exactly what to do to crush your first 5K, you can focus on enjoying the event without killing your body in the process. If you're looking for a deeper dive, you can check out an article I wrote called "How I Avoided Gut Rot, Ate Pure Corn Starch, And Won The Leadman Triathlon" where I discuss all the nitty-gritty details of race-day nutrition.