@jmadigan001 I'm pretty darn close to you in age, and let me say right off the bat; thank you for seeking out the advice of our elders! Take what they say to heart, I guarantee you will quickly see yourself getting better and more confident.
Now, the key to being taken seriously, and warding off a load of issues, is pure and simple professionalism. Now, don't interpret that as me accusing you of lacking it, crap, I just remember how unprofessional I was when I first started out compared to now, and I still see ways I need to improve my game management. I know it's a cliche, but it's true. So, based on your OP and remembering my experience from being your age a couple of years ago, I would recommend you do the following:
1) Remain the calmest and most even-keeled person in the entire complex. Nothing good ever comes from getting angry, even when it's totally reasonable to be. It also helps to either de-escalate the situation or make the guy yelling at you look like an unhinged fool.
2) The plate meeting is crucial. Take the time to write out the main points you need to cover (rule set, equipment check, etc.) and then practice reading it. This, and debate, did wonders for my stuttering and "uhh...ummmm"ing at the plate meetings.
3) Remember the 3Ps of ejections: Prolonged - Repeatedly bringing up past calls and refusing to cease and desist after saying their piece. Personal - Anytime a coach says "you suck" or "you're a disgrace" dump them. Profane - If a player or coach lets loose a four letter word, they're done. Keep an index card and pen on you or in your field bag so you can jot down what happened.
I realize this is going to be imposing, and even a little scary if you're solo, but you are the authority on that field and need to be able and willing to bite back when someone crosses the line. Now, obviously, don't go dumping everyone because they look at you crooked.
4) Don't appeal everything to your partner just because the coach asks/demands. If they ask you to check on a swipe tag/pulled foot, that's one thing, but don't ever appeal a tag play at second to your partner unless you know with total certainty that you blew it. Appearing weak in the face of coaches arguing may buy you some temporary respite, but things will only go down hill from there, b/c now they think they can go umpire shopping for the call they like.
Another thing on this: if your partner makes a call and the coach comes to you, the only thing you that should come out of your mouth is "Sir, you need to talk to partner, he made the call." I made this mistake early on, thank god, but it was indeed embarrassing to be getting gently shredded by someone I respected after the game.
5) LOTS of younger players see young umpires like you and me, and will often get chummy. Be kind of course, but professional, otherwise you'll start running into issues.
6) Find a training camp or clinic! I started out as an in-house rec umpire, and the training I received was very, very, very basic. Going to a camp or clinic will help you immeasurably, especially when you get behind the plate for kid-pitch. I'd recommend looking up free clinics run by your HS association, MLB camps in major cities (!!!), and maybe your region's Little League. And of course, keep asking questions here!
7) Hustle. Again, cliche, I know, but if it looks like you're trying to get in a good position to make the call, that looks really good on you, and helps your street cred on close calls.
8) Have some freaking fun man! Baseball is the greatest game on God's green Earth, and you get the best seat in the house!
Edit: Found a great quote from @ElkOil on one of my first posts on the site.