Wear what you're comfortable in but if you're going quite casual, then it's good to dress up the outfit a bit with attention to your makeup, shoes, bag & general accessories.
Tips:
1. Many clubs do not look kindly upon trainers. Footwear doesn't have to be high heel, but it should at least look fairly smart.
2. Clubs can get Very hot inside (so only bring a jacket if the weather actually calls for it on the journey to or fro from the club).
3. Bring a bag with some essentials such as money specifically reserved for a taxi if you need one (figuring out how much you would get charged for a taxi back home late at night as they can get more expensive over the course of the night) andany essential numbers written down on a small piece of paper (were you to lose your phone etc). I have seen SO many girls over the years spend their ride home money or get abandoned or lost from the friends responsible for helping give them a ride home (etc), it's ridiculous!
Sometimes it's just good to have that ability to be able to pay to go home early if you feel the need to. NEVER SPEND YOUR TAXI MONEY.
While you don't want to travel too heavy (big bag getting in the way, lugging around an unecessary jacket etc), you also want to be prepared for some basic potential situation stuff were it to happen. In terms of what bag to bring, REALLY focus on how easy it is to pick pocket (not that such crime is usually an issue in night clubs, but you also don't want to go inviting any unwanted hands either! Some bags look very nice but are very lacking in terms of keeping your wallet & phone secure).