Pilates and Zumba will add significantly to your core strength. I can't promise the visual six pack. I see people in these classes who are strong as heck, but the muscles are covered with fat. They look awesome and are powerful though.
I started out skinny and with a flabby stomach that I could not pull in for more than a few seconds. I had overdeveloped upper abdomen cough muscles, but less muscles lower down, on my sides and in my lower back. Now my back, sides and stomach are pretty ripped. My coughing is more efficient now too. People often say, "Bless You" when I cough because they think I have sneezed. I think it is b/c I can generate a bunch of power (and goo noise) with one cough. That might be coincidental though.
I do 3 hrs a week and I have no idea how long it took. I am about to start year three of being a bit of a gym rat.
Okay, I just saw that you are a 15 yr old BOY. Well, you have to decide your priorities--abs and possible guy-embarrassment...
There are only a few men in my pilates and zumba classes. Pilates is completely asexual, but guys tend to think they can't do it. With regards to Zumba, guys modify some of moves, like for belly-dancing, to be more dude-ly. The hip hop, regaeton and salsa components are not feminine by nature.