Landlords want three things from bar staff. Turn up on time, be nice and helpful, be honest. Simple.
OK, if you are new, no-one epects you to know everything, but they are not mind readers, so, if you are unsure, ask. They'd rather tell you, than you guess, wrongly.
When you are being shown something, pay attention closely, and if you don't understand something, then ask for it to be shown again, don't worry about seeming stupid, everyone has to learn, and you learn by watching, listenng, and asking.
Make sure you are not standing around doing nothing. If you have nothing to do, ask the person who is training you what to do next - nothing annoys employees (and bosses!) more than the new person standing about staring into space while they rush around.
Be nice to absolutely everyone - you won't like everyone, but they are established, and you are new. On the same principle, keep out of gossip, and you will hear lots, just smile and say nothing. If you get involved and upset the wrong person / people, you could be having a very short career, or a long uncomfortable one with all the lousy jobs suddenly landing on you.
Just be yourself, be keen to learn, don't be too chatty until you get the vibe that goes on, and get to know your colleagues. Pub work is all about being sociable, but not at the expense of getting the work done.
If you make a mistake, own up and apologise. If you get an order or change wrong, apologise, explain you are new, and smile - being nice will get through almost all situations.
Enjoy it! Don't worry about anything, you are there to learn, and you will, and once you;ve got the hang of it, you'll start to feel comfortable.
Above all, if you are relialble, nice and honest, you represent solid gold to any landlord, so you can always get work, if you don;t like where you are,move on.
Have fun!