a cracking voice is just a normal part of the entire voice changing process. it sure does take awhile and it can be embarrassing at times, but it will go away on its own. when exactly that happens is almost impossible to say. every guy is different and develops differently. you can try speaking just a hair softer than you do now and see if that helps your cracking problem.
its changing this process takes a different amount of time for every guy. just give it time and it will stop cracking. in the mean time down worry about it to much yeah it can be embarrassing but try to stay positive.
There's nothing you can do to prevent the changes going on during puberty. The good news is that they're temporary. However, you noticed that runon sentences your voice is more prone to cracking. You could make the effort to eliminate runon sentences (your English teacher certainly wouldn't mind!) and perhaps breathe more when speaking. That, along with time, should cut down on your voice cracking.