I'm 15 years old and I weigh 175 lbs. Should I be concerned?
Not enough information, If you are 6–4 you would want to put on some weight and if you are 5–4 you might want to see the doc to see if you need to lose some; but then you may not have started puberty and your body is going to need some fat and will change a lot. Or you could be 8% body fat at 175 and you should be amazed that you are in such great condition.
If you are "overweight" and not in a healthy way as you question seems to suggest as I pointed out it still depends on where you are in your development. My best friend was over 6′ and 129 lbs. at 15. If he was 175 lbs he'd have been pretty solid and appropriate.
I would suggest if you haven't been to the doctor a checkup is in order and you can discuss your concerns with him and if you are in need of weight loss (and for teens I usually think it may go away naturally as you grow into your body) he can help you figure out how to do that well and how to build your body at the same time.
One of my favorite examples is a singer named Brian Justin Crum who was on America's got talent a couple years ago. When he was a teen he was overweight and got bullied for that and other reasons. If you look at his performances you'll see a powerfully built guy who doesn't look like he has ever had an extra pound (and an incredibly powerful singer too...). But in his audition video they show photos of him as a somewhat overweight teen.