So your 18 with a small hole you say 0.5cm, thats sounds really small, just how did you measure it, and you have never masterbated as this may help to incress youir size, no bf who can do it for you, its all down to relaxing will help.
Good Luck
The human body is made for intercourse. You are a virgin so naturally your "hole" will be small. Once you are ready to start having intercourse, your body will adjust naturally and you will enjoy it:)