Dear Camille, 

If your father wants you to wait it would be good to ask him why. There could be one or a variety of reasons behind his opinion (perhaps he sees something you don't, or maybe he thinks your decision is not matured, or it might just have taken him by surprise, or he does not know the group you are thinking of joining... or, in an extreme, he may have difficulties with the faith that blind him to the 'worth' of a vocation). I do not know where he might stand, but you have to try to pinpoint the problem and address it. What you do concretely will depend on the actual situation. Sorry I can't say anything more concrete, but there is one thing you should do for definite: pray. Pray for clarity, charity and generosity for yourself, and the same for your father.