If you have to ask then you're too young to be kissing girls. For now just focus on your schoolwork, that'll one day make you a gainfully-employed, highly-desirable man and women will be fighting over you.
That's a hell of a complicated question. You have to recognize the situation when it presents itself then act. Simplest answer I can think of is to take her on a great date and move in when you are saying goodbye before dropping her off.
Honestly since you have to ask the question when is the time to do it? that means that you are not ready for that yet. You should wait until you are older to do it. It would be best to do anyways you will not regret it you will be happy that you waited.