Help! Military Ball!!

The guy I'm talking to is in the Air Force & has asked me to go to the Air Force Ball with him. I've never been to anything like this and I have no idea what sort of dress to wear. I know there are "rules" to this and I don't want to look out of place. I asked him and all he could tell me was knee length or longer which is similar to what I found online. I just don't want to wear something too much or too casual. Thanks ladies!!