Try and keep track yourself of order at the bar, always reassure customers "I'll be with you shortly, just serving this guy" It stops people getting frustrated if you miss them.
You'll probably not need a bottle opener, most bars have fixed ones, and generally wet sales are draught in the UK.
If you notice the barrel has gone, turn the tap off ASAP. If you get too much air in the line it'll take ages to sort, and you run the risk of pissing off everyone waiting, not just the person you're serving. If it doesn't look right, stop.
DO NOT ATTEMPT TO POUR A GUINNESS SHANDY!!! Someone thinking they're clever spotting new staff will try it. IT DOESN'T WORK AND MAKES A MESS.
Most importantly, try and enjoy it. Keep calm, keep smiling, and you should be fine. It's a fairly simple job, it's hard to over complicate it.