When you have an invitation to a Cocktail Party what do you wear exactly?

It has a 7.30pm start so it's not afternoon..it's evening...

It's not smart casual..it's specifically cocktail....

Would pants still be ok for a lady to wear? Or should i wear a very formal dress? i do not wear formal dress before , i usually wear simple t-shirt and pant , really do not know how to choose formal dress , my friend have bought many karen millen dress from : www.karenmillen-coats.com before , so i want to try karen millen dress,can you give me some suggestions which dress to choose? And for a guy - would they wear a suit?Where i can buy the most nice suit?

What do you think?