A bit of eye contact helps. Some dudes just go in and that works for them, but I like to atleast make some sort of informal contact via a smile, a nod, wave, something. If she acknowledges it, and reciprocates in kind, you go in.
Now, you up there, what you gonna talk about? The weather? You can be either direct with the shyts or indirect. By being direct, I mean you should pretty much start asking questions you want definite answers to upfront like "What's your name?" "I seen you all night and I like the way you looking in...whatever the fukk she's wearing." Or "You the X girl I've seen wearing that shyt, but ain't none of 'em pulling it off like you." type shyt to let her know that, 1, you seen her...you been looking

, 2, you shooting your shot, she's being chat up. If she's with it, she'll roll, if she not, play it off, cut your loses, tell her "Pardon" and keep it moving.
Or you could try my route, the indirect route where you go over and ask her some seemingly benign question to get the conversation going like If she drinking some fruity shyt and you got a beer, you can ask her where she got it from, you thought all they was serving is beer or some bullshyt (Make sure the answer to your question ain't painfully obvious and you get called out on it.) or you can make up some sly excuse about how one of your friends told you her name was (Make some shyt up) and when she's all like "Nah, that's not my name, my name is...." you apologize, tell her she got a nice name, ask her what she's drinking....keep the conversation going.
Ain't hard approaching chicks, breh, it's just to keep 'em interested in the conversation if they ain't feeling you directly off the bat. I usually just lurk and observe and try and catch 'em when they go to re-up on alcohol and make small talk there. Sometimes if the liquor has me nice, I walk up to 'em, ask their name and hit 'em with the "Man, I was tired of watching you from afar, wondering what the hell to call you in my head, so I'm asking...what's your name, Beloved?"
A bit unorthodox but I'm eccentric to say the least. Best of luck to you out there, breh.
