How to tell my dad that I don't want to play baseball anymore?

Well, I'm thirteen and have been playing baseball since I was nine, I've always been really good at it. But this summer I went to the gym with some friends and I was hooked, I loved how you could get in better condition without it bieng so competitive after a while I started to hate baseball, and I just need to know how to tell my dad I don't wanna play anymore, I've already made my mind up and I'm done with baseball.