Confidence is all about body language. If you look confident then players will assume you are and will accept your decisions more readily. When you go out to start the game, make the first whistle loud when you call the captains in. That's their first impression of you and a good loud whistle implies confidence even if you don't feel it.
If at any time you are feeling uncertain then make all signals really big - for instance, if there's a throw in make sure your arm is straight out and hold for a second. That says, "I'm sure about this, so don't argue with me!" A quick point before dropping the arm says "I'm not sure so I'll give it that way but don't pick on me if I'm wrong, ok?"
There's no need to signal immediately for throw-ins, goal kicks / corners etc. Give yourself a second to think about the decision before a good clear signal. That has several benefits: if one team goes to get the ball and the other moves away to mark then it's an easy sell to give the throw their way and makes it less likely you'll have a brain freeze and signal the wrong way. If both teams appear to be expecting the throw then it allows you time to think about which way it is. Delaying for too long can make it look like you're going with the shout, so don't delay more than a second or so but try to avoid rushing to signal instantly. If you go the wrong way this can cause uncertainty next time, leading to loss of confidence. It's very rare you need to signal immediately, and this is normally only when there's going to be a card for a serious foul. This is highly unlikely to be an issue in your game.
When you whistle try to ensure it's a confident blast rather than a quiet one, and try to make the whistle "talk" This means, if the ball goes out of play and you need to stop play then a quick blast is all that's needed. If it's an offside then a bit louder. A foul is a louder blast and if it's a bad foul then a really loud blow tells the players "I've seen this and I'm going to deal with it!" Try to avoid double taps with the whistle to stop play.
If you do make a mistake then don't worry about it. No one will die because of it. If someone questions a decision I find that saying something along the lines of "it was a foul (or whatever) from my position" can calm it down, because unless they're in exactly the same space as you then they can't know for sure.
I wouldn't worry that you're going to get loads of grief. I find that most games are perfectly ok and nothing happens at all. You won't get everything right, but you certainly won't get everything wrong. At the end, you may be feeling that you did get everything wrong, even though you didn't. In this case, make a list of stuff you think you got wrong and then a list of things you did well. The second list will be much longer than the first.
Remember, it's all relative. What may seem like a massive error is actually nothing when taken in context. Several years ago a good mate of mine - who unfortunately died over Christmas - refereed our County Sunday Cup final. At the end of the game the losing manager told him "you were awful! you made loads of mistakes. You've got to get these decisions right in the big games!" Mick didn't tell him that a couple of months previously he'd been on the line for an FA Cup last 16 game at Elland Road and he had indeed got everything right in what was actually a big game!
In your first game you won't remember any of this, because you'll be too busy trying to remember that you're the Ref! In my first game I appealed for an offside(!) before remembering it was my job! Good luck, don't worry about it. You'll be fine.
.. Good luck to you all in your first games!! 