Dear Kathy,  

The only way to find out if your desire is what God wants is to take steps to find out. I would guess you are already thinking of some group specifically that you would like to join, or have some nun you really admire and would like to be like her. The good thing about your question is that even though you desire to be a nun, what you really want is to do what God wants.

As regards your situation, keep in mind that not every desire that comes into our heart is necessarily from God, even if what we desire is good in itself, like being a nun. Sometimes it can be just infatuation, or it sounds romantic and adventuresome, or it may be an escape. How can you tell? Ask yourself why you want to be a nun. Honestly. Ask yourself if right now you are doing all you are supposed to (as a student, daughter, etc... Also, do you pray, find time for God) because that will show if you are really looking for God's will or not, it will show if you are following your whims or if you are capable of doing your duty instead of running away from it.

You also need to speak to someone who knows you and whose advice you trust. The above should give you a lot to go on. Now, it is also quite possible that God is calling you but your behavior might still a little immature. If that's your case I think you know what you need to do.