I'm not sure if you're male or female, but I am guessing you're male. It's normal for teenagers to come to orgasm/ejaculation fairly quickly. It's a new experience and can be very exciting.
What you can do? Well, sexual activity doesn't have to come to an end, and it doesn't have to involve an erect penis. I can't be explicit, but I'm sure you can use your imagination and ingenuity.
Some people find that masturbating earlier in the day can help them 'last longer' during sexual activity. Whether you do this or not, it's not anything to worry about, simply personal preference.
"And when the answer that you want/Is in the question that you state/Come what may..." - Coheed and Cambria, "Blood Red Summer"