Hiya, and welcome to the club. As the others have said, confidence will come with practice: the more you ref, the more comfortable you will be. That being said, I'll give you 5 points of advice that are quite basic, as you're probably looking for concrete tips. Don't focus on all 5 at a time: each time you go out on the field, try to give yourself one thing to think of (which can be really basic - these 5 are examples of that).
1- Practice your whistling off the field. Whistling properly (with a Fox 40, for instance) takes practice, and you will seem and feel instantly more confident and more respected if you whistle properly. With a Fox 40 (or any pealess whistle), you purse your lips and try to blow as sharply and narrowly, almost like spitting (or playing a woodwind instrument).
2- Make all necessary signals - remember which those are, and practice them in a mirror. It does make a difference whether a signal is done lazily or sharply. You are a referee, and communicate primarily through your whistle and signals.
3- You don't need to be in a hurry. Take a second extra and think through the correct decision. For instance, on a throw-in, wait a second, decide who gets the ball, and make your signal, in that order, instead of hurrying to make a signal, realising you're wrong, and switching sides. The players will accept you taking your time and being right, and prefer that too indecisiveness.
4- Try to think about whether you are in the best position to take the most probable decision. A good position has two main criteria: 1) it offers you a good vision of the play (and where play is going, if you can); 2) it is adaptable, meaning it allows you to change quickly if play comes your way, or if it moves quickly. So, for instance, you want to avoid getting caught along the touchline (since you can't move if the ball comes your way), particularly if the ball is on the other side (you can't see if the ball is out on the right touch line if you are close to the left one).
5- Take the habit, at every stoppage, to look around the field (if you can). Coaches will appreciate if you acknowledge when they want to sub, and it will avoid a lot of anxiety for everyone if you spot injured players early (especially with kids); parents tend to get nervous when players are injured. And for you, it reassures you that everything is fine, and that everyone can concentrate on soccer.
As for your feedback for yourself, I'd advise you to put it in concrete terms. Not "this game was crap" but "I did not position myself properly". That way, it gives you clear things to improve for the next game. Similarly, if you do a good game, focus on drawing out 2-3 elements that worked well and that you can reinforce again (and then, when you feel a game is going badly, go back to those 2-3 positive points).
Most importantly, enjoy your time on the field, and ask for advice from other referees, referee supervisors, and here.
