How do you approach girls at a bar/club?

New to nightlife. For the first couple of nights, I wandered around the bar/club alone looking for a girl or pair of girls that were also alone. Well, I probably look like a creep doing this because rarely do I find attractive girls that aren't already being talked to by a group of guys. So usually I just end up talking to some other lonely dude(s). Do you guys just barge into random groups and interrupt their conversation? Do you cold approach, sit at the bar, or stand around and wait for eye contact? Are there certain groups you tend to approach vs not?