Be aware that some guys will try to spike your drink (drug you). don't leave your drink under anyones control. In fact, I say don't drink... drink water, or only drink mildly so you have your wits about you. You can always make whatever choice you want and have just as much fun.
You may get hit on even if you are with someone else.
Wear whatever fits how you'd like to be perceived. If you're looking to meet a guy to hook up with, wear a skirt and/or skimpy top. If you're going to mainly hang out, especially if you're kinda with someone already (like in your case), some skinnies and a v-neck should suffice.
Depends if you are going at night or during the day. During the day i suggest a sweater type deal with jeans and boots or comfortable shoes. During the night I suggest a flattering top with a leather jacket and dark jeans and boots.
You have never been to a bar before? Do you live under a rock? I wear casual clothes, sweats and a t-shirt. for a date probably nice jeans, and a nice shirt. nothing fancy, a bar is a laid back place. Know what you want before you call the bartender over.
Something skanky. If you're planning to get a little sloppy I wouldn't do the heel thing (even though they're hot) just because you're most likely be falling over and look stupid. But in general less is more; but stay within your comfort zone too.