Your normal bud, you are probably young and active. Just make sure you have control over your emotions. True enough just as the other fellow said, try masturbation.
Well, you can practice not getting hard when you're alone. For example, any hot girl u see in a magazine or on tv, just look at them as regular ppl. You could also try thinking about something else, or something tht wont get you aroused when around hot girls. But tht is something u should work on. It could be quite embarrassing on your part.
It's called getting an erection. It just means that you are a really horny young man. Masturbation is a completely safe thing. You should consider trying it. It will help you release a lot of sexual tension and will make you less horny.