Everyone will have slightly different advice. I've run about 180-200 5k races over 20 years, and have developed a pattern that works for me. You will eventually do the same. Take little bits of everyone's advice that rings true to you, and make it your own. Here's my contribution:
Firstly, warm up well. I don't know what your weekly mileage is like so I am not sure if this would be suitable for you, but I always jog the entire course before the race and envision what certain parts will be like while I mentally and physically prepare. If you are a low-mileage kind of runner this may be too much, but I would still ask you to ask yourself how many times you have gone out for an easy 10-12km run and felt great half way in, blasting the rest once you've warmed up. Jogging the entire course is not a waste of energy unless you are really new at this. It sounds like this is your first 5k, but I don't know how long you have actually been running for. If it does seem like too much, I would at least run the last km or so in reverse then turn around and run it in, doing the envisioning I mentioned earlier. I like timing my warm up to end about 5 minutes before the race starts. This is a personal decision. You certainly may want more time (many do, I'm a little different with this aspect), and there's nothing wrong with that. You'll also have to find out if you like to be alone while you warm up, or socialize and get energized by chatting with others. I can do either, but for very important races, I prefer to keep to myself.
You may see people doing short strides right before the gun. This gets the heart rate going, and makes the start less of a shock to the system. Nice smooth strides in which you slowly accelerate for about 10-15 seconds are ideal. I usually do about 3 within minutes of the race start.
km #1: If you are going to be wearing a GPS watch of some sort, pay close attention to your pace for the first 2 -3 minutes. Everyone goes out too fast. Literally everyone. You don't have to make that mistake. I love letting them go, knowing I will reel them in. I completely focus on relaxing as much as much as possible and running as slow as I can without endangering my goal time. If you do it right, the first km will feel like you stole it, and it's only a 4km race. If you are shooting for 19:59, go out at 4:00 - 4:02 pace right from the start. NO faster. Aim to hit km 1 in 4:00 at the very fastest. 4:01 would be ideal, in my opinion. 4:02 isn't bad, either - better than 3:58, if your goal is 19:59.
km #2: This to me, is the first km of the actual race aspect of it all. I start to pay attention to the people around me; who I want to avoid (heavy breathers, it bothers me), and little packs I might want to stick with. It is always easier to run in a little group than alone. Always. I will try to find one that is running the appropriate speed, and run with them. I maintain pace (for you, now 4:00) and concentrate on relaxing any part of my body that is tense. Often it's the shoulders. Shake your arms out if needed.
km #3 and #4: This, for me, is when I start to feel the true onset of discomfort (km #3). Not a lot, but it's present. A lot can happen during these two kilometres. This is the meat of the race. You're hurting a little, but the end is too far away to think of. Often the little group you are running with starts to disintegrate. It's time to start making your move. Look ahead, and start finding people to pass. Slowly work your way up to each one. Once you reach someone, you need to relax for a moment because you probably had to pick up the pace to make contact. So run with them for 20-30 seconds, and move on to the next person. Repeat. Keep pick people off. If you are feeling good, it is during this period that you will gain a few seconds. I would aim to run km 3 and 4 slightly under pace: 3:59 each would be excellent. No faster. There's a huge difference with each second faster you run when you are running at your limit. For me (when I was fit) the difference between 3:25 and 3:23 pace was massive. I could blow my race if I wasn't careful, and end up limping in.
km #5: This is it, baby! You are hurting, and you spent the warmup knowing this moment was nearing. I have always noticed that no matter how bad it hurts, about 10 seconds after I cross the finish line, I am already recovering and knowing I could have ran that little bit harder. You will feel like that too, so give it everything you have, and more. A little trick that works for me, and I know it sounds silly and dramatic, is to pretend I am in a movie, and my "part" is to run the race in x time, or to beat the person in front of me. Other people concentrate on the heel of someone in front of them. Still others visualize a rubber band attached to them and the person in front, pulling them forward. All these tactics are simply ways to distract yourself. Another thing Ido is concentrate on relaxing every part of my body - my face, my neck, my shoulders - everything. Elites are awesome at this. Watch a YouTube video of David Rudisha running the 800m at the Rio. His face is expressionless. He is a master of relaxing during what is probably the most painful distance to race. Tensing up just makes matters worse, and great runners know it. Try to be as calm as possible, while your heart feels like it is going to explode.
When the bear jumps on your back with a minute to go, focus on taking little, quick steps. Newbies overstride like crazy at the end of 5k races. Don't do that. Shorten your stride and increase your turnover. You WILL go faster. It's okay to pump your arms a little at this point. Just get to the line any way you can.
Sorry about the length of this post. I hope I helped - good luck. I want to read a good race report!