Kissing should be a shared decision. You need to have permission to French kiss someone, but when your lips are locked with theirs you may not want to stop and ask, "Hey, this is great, but can I put my tongue in your mouth?". Open your lips slowly and just a little during the kiss so that one of your lips is sandwiched between theirs and one of theirs is between yours. As you are locking and re-locking lips, brush your tongue against your partner's lips ever so slightly. This should make it clear that you want to French kiss. If your partner's tongue does not respond in like fashion or if they pull away, you will have to save the French kiss for another time when you are both ready.