your anxious partly because you've already in your own mind placed an expectation of failure on the outcome.
first of all, don't approach any woman who hasn't looked at you or given the slightest inkiing she's interested in you. if a girl hasnt had the opportunity to notice you, give her that opportunity by walking by her. if she likes you she'll check you out initially and then a little later on- if she likes you she'll know exactly where you in that club at all times. you dont have to keep trying to be noticed once she first sees you. if a girl is in your vicinity, and she can see you, you'll know if shes into you. she'll look over at you. enough times to let you know shes interested, but not enough to look desperate.
now, you have to let her know that your interested too without looking creepy. dont stare at her or smile like 'hey is see you! wow your hot!' that screams desperate, just let her see you looking back and thats it. and technically you dont even have to let her know you see her checking you out. then go to the bar and get a drink. one of two things will happen now when you go to the bar for your drinks, she either go to the bar herself and stand beside you, or put yourself near enough to her to say hi. if she puts herself near you to say hi, dont just barge up to her and say hi- gotta be tactful and see who shes with- if shes with a group and conversing leave her be until she separates herself from the group for a bit and stand off to the side, which she will do eventually.
then just say 'hi' thats it. this isnt rocket science bro. if she digs you she will help you carry the conversation cause she wants you to be there. if shes not digging you you'll feel like conversation is a struggle. the key is to leave the conversation before you both get awkward, and when you leave dont close the conversation just leave it hanging on a good note cause you want her to want you to come back and not have it awkward to resume the conversation. you dont have to stay attached to her the rest of the night.
thats all there is to it dude. if you need directions after this point theres nothing i can do for you as getting to this point is tough part for most- the rest from this point on should sell itself. dude, just be yourself, dont try to be a certain way- relax and let shit play out. if its meant to be it'll be. if not nothing lost.... thats it.
i should be charging you for this shit.