Formal dress?

Hi ladies! So I’m going to a very fancy party in a couple of months. Everyone who attends must be in formal wear so either a tux or a formal dress. However, I don’t know what to wear! The only thing formal that I’ve ever worn is prom dresses back in high school. Do I just go look for prom dresses? Where do I look? I’d love any suggestions and advice! Do I get a long or short dress?? And what kind of hair and makeup? I’ll be doing it all myself so I’m completely clueless! I could really use some help! Thank you!