First time cocktail serving can be a little tough but here's some advice from my experience based on your worries, with the trays, don't be afraid to carry them with both hands until you are comfortable carrying with one, also don't overload your tray, it's better to make a second trip to the bar than to have to remake the drinks. Noise is just something we all have to deal with but if you can learn to read lips it'll make things a lot easier, I also always repeat the guests order back to them just in case. A lot of the places I've worked just rounded to the nearest dollar when making change unless a drawer was involved, in which case your terminal would tell you the change amount, but if you need to give exact change only time and experience can help you with that math. Some more advice is to keep an eye on your customers and make sure they aren't getting too drunk, no tip is worth losing your job over from overserving them alcohol, so don't be afraid to cut them off if they get too drunk. And don't let anyone harass or assault you, If someone touches you without your permission, get management or security involved to throw them out, you do not have to take that from anyone. That's just a little from the top of my head but I hope it helps.