Mango Elephant
Deity
Well she would not think I'm being serious.
I'm 14 too, and if she won't think you're serious this either means
1. She's immature and you shouldn't care what she thinks
2. You're personality is one that leads people to believe sarcasm, which is a very bad trait and you should work on improving that
-or-
3. You've misjudged her reaction, which is what I believe is the most likely. Even if she thinks you're kidding, she'll still laugh or smile because that's what you're supposed to do when people try to make a joke, and she'll like you better for it.
Apologise is still what I think is the best way (unless 1. is true) because if 2. is true than you can work on improving your personality. I don't know the full context of the situation, but I've found that generally being honest is the best way to live, and if you're sorry for aggravating her, than appologising would be the most honest thing to do.