Sweetie this is the question you should ask with yourself. You need to be ready for sex emotionally, mentally and physically.
Emotionally, you'll know you're ready for sex when you love a boy so much that you want to be naked with him, giving him all of yourself and most of all getting as close to him as possible;
Mentally, you should be mature enough to not just go with the flow, but really think about the pros and cons, looking into reasons why you're willing to have sex, finding out and taking into account the risks, deciding on contraception, etc. You should be mature enough (health and life) before sex.
Physically, you should at least have passed puberty: ie, your body tells you it's grown up enough for sex by looking shapely, hairy, your periods have not only started, but also have become regular and reliable enough for you to be able to recognize when you'll be fertile and when not. So in your most fertile days you can keep yourself away from sex.
When you hug, kiss and fool around with your boyfriend, your body will react ready when it is, by enjoying his touch, your nipples will stiffen, your vagina will feel wet with discharge (getting ready for possible penetration)... This is important, because children tend to just need to pee, whilst an adult body will actually get ready for sex during foreplay (ie- kisses and cuddles).
First time sex hurts for a girl because the hymen, a flap of tissue that seals your vagina, is stretched and torn during penetration. This only happens once, but you might re-experience pain if you're not relaxed and wet enough.
If your body is mature but you don't get wet during foreplay, that means you're scared or nervous about what's happening... ie-not ready! In this case, you should talk to each other and establish your mental closeness before your bodies get closer.
Hope it helped to answer your Question.