My main concerns revolve around remembering everything in the midst of a hectic game; that and making a major gaffe, like getting the score wrong, or stopping the clock an forgetting to restart it!
Welcome to the forums, Andrew - and more importantly, welcome to the wonderful and rewarding world of refereeing. It's an extraordinarily steep learning curve - OB's advice about making notes regarding every game is very sound.
You are right to be concerned about the things you mention above, but there is no realistic prospect of avoiding these things. You WILL make these mistakes - I'd bet my mortgage on it. The thing to work on is how you react to the realisation you've made the mistake, as well as things you can do to mitigate the mistake.
On losing track of time: before the kick-off at the start of each half, make a note of the time you start the game; AND make a note of what the time will be 44 minutes after that. If you lose track of time, blow for the end of the half when the 44 minutes are up. You won't be far enough out for anyone to be able to complain.
Make sure your scorecard has space for you to put a mark whenever a type of score occurs -so you have a space for tries, conversions, penalty goals/drop goals. (you won't see the latter for at least 4 years, so it's perfectly acceptable to use the penalty space, and differentiate the DG by (for example) putting a ring around the marker. This way, you have two methods of scorekeeping: the individual scoring events, and the running total. Hopefully, both will agree at the end of the game; if not, you have the ability to check with any interested spectator; and if there are none of those, I generally find the events tally more reliable than my running totals.
As mentioned, you WILL make mistakes. Before patterns of play imprint themselves on your mind, the brain is spending time processing what just happened, only to find that lots more has happened while the first batch was being processed. In time, it gets overloaded, and you tend to shut down somewhat. That's when you need a pre-determines strategy to get though the game. Its all too easy to decide that you simply don't have what it takes to be a referee, and to quit despondently. In fact, all you need is time to overcome the errors, and to give yourself this time, you need a strategy to get the players off your back. The best advice I can give is this: even if you are convinced that the players are fully justified in moaning at you because you are crap and are adversely affecting their game, there is one Law you can enforce uniformly even when you are in overload. That law is 10.4(s), which warrants repeating here:
[LAWS](s) All players must respect the authority of the referee. They must not dispute the referee’s decisions. They must stop playing at once when the referee blows the whistle except at a kick-off or at a penalty kick following admonishment, temporary suspension, or send-off.
Sanction: Penalty kick[/LAWS]
If the players are moaning at you because they think you are not properly enforcing the LoTG, they cannot complain if you invariably and accurately enforce this one. If you follow your heart and let this one slide because the moaning is justified, your day will go from bad to very much worse, and there can be few of us on this forum who have not come off a pitch in near despair, and seriously considering quitting. Most of us, reflecting on our (crap) performance, then recognise that the game really started getting away from us when we failed to use 10.4(s), allowing an increasingly acerbic drip drip of negativity to pervade the pitch.
Good luck with the refereeing. You will cock up. You will embarrass yourself. And you will feel that you have no right to be wielding a whistle. The key thing to remember is that after 8 weeks of hard work and self analysis, patterns will have imprinted themselves, the arm will go out for advantage without you having to think about what the offence was, and you will be on your way to fulfilling your potential as a ref. You just need a dose of reality to help you get through the initial period when you are all at sea.
Let us know how you get on.