What to wear on a party always becomes a subject that can make you totally lost. If you have already known what might look good on you then there will be no problem. But if this is your several times going to a party with inappropriate choice then it is normal for you to feel insecure.
Long dresses can be a way out for your problem. You can try to wear this kind of dress in a party. Most women love to wear long dress since it will make them look elegant. It is just the general look that you can add with some designs creation to get additional look just as you wish. For example if you want to look sexy and elegant with your long dress, then you can make it into a one sleeved dress or even give a slit from your toe to thigh. Just remember not to make it too high which will only make it looks trashy instead of sexy and elegant.
Many other styles can also be your choice depends on how you want your dress to look to other people. Once you have known the image then you can easily create your long dresses based on that.