I agree with the bar talking; it's easy to bring up a conversation topic with stuff going on, it's a bit quieter and it's easier to start as you're standing close together.
I think in general, guys enjoy talking to girls so just say hi and ask something simple; I don't know any guys that'd just ignore you or tell you to get lost as long as you weren't like all hyper and squealy or incoherently drunk, so you should be fine. You don't need to be particularly forward; just small talk and if the guy's nice, you can move forward from there.
If you are going to do that though, be careful about meeting some sleazy guys; some think a girl looking at them means they want to sleep with them, but by staying 'unforward' and noncommital to start with, you should avoid that kind of situation.
Not sure if it's too off topic to be useful advise, but maybe if you are looking to meet nice guys just to chat with and swap numbers, a bar/club would probably be one of the worse places to look; loud, lots of alcohol and typically a place that meetings between males and females end in pulling, which isn't what you want. Perhaps a coffee shop or cafe or something would be easier, if you are particularly interested in meeting someone. If it's specifically bars you want tips on, ignore that last paragraph! Good luck.