Look around for openings at the bar where you can stand to order. It's customary to queue up behind the people already waiting, even if there are small gaps between groups. Don't try to squeeze into a small space or reach around people to get the barstaff's attention but do fill the full space of the bar where it isn't awkward to do so.

People are served in the order they arrived at the bar. The barstaff keep track of who has been waiting the longest. It's polite to allow people who arrived before you to order first, even if you end up at the front of the bar. If the barstaff try to take your order before someone who arrived first, just gesture to that person and say something like "I think they were next".

If it is busy once you have your drink step away from the bar so the next person can take your place. Cheers!