What type of dress should I get?

I am going to a military ball- a dance. I don’t know what color/type of dress to get. It has to be modest- nothing that shows cleavage and has to be close to my ankles.I have never been to a formal dance so I don’t know what the typical attire would be for this. Any suggestions?

I am going to goodwill and St.Vinnies (thrift stores) to look. I don’t have enough money to go to a mall.