You can't stop your voice from cracking. Your larynx (aka voice box) grows bigger. The reason why your voice is cracking is because your body is getting use to the changing size of his larynx. Luckily, this will only last for a few months, or maybe a bit longer; it will go away though. There is away to avoid voice cracks, but of course you can not stop them. I suggest you drink plenty of water. It helps keep you dehydrated and helps you speak more comfortably. Second, speak slowly and deliberately. When you rush your words, your voice tends to crack more. Now, you should relax when you are speaking. Your voice gets higher when you are either stressed, or tense. These are just the little things I know about helping your voice stop cracking.
I hope this helps you~ Good luck! ^-^
