Gynecologist

I am 15 and sexually active. My parents are not aware that I am sexually active because they are very religious and would kill me if they found out. I was just wondering when should you start seeing a gynecologist, when you become sexually active or is there an age, or just when something seems wrong? Don't have any problems down there that I'm aware of just not sure if I should be getting it checked out and if so how often.