One of the devs bought this to my attention.
Casting is much more than just blabbering about the game. But blabbering about the game intellegently and coherent is the most important core skill you need to have.
I would highly recommend ReDeYe's guide to broadcasting as a place to start of. It's a nice small read on some important aspects to broadcasting.
Secondly, cast a lot. Whatever games you can find. Could be 2 teams just scrimming on SoStronk, could be some random ESEA Open game, hell it could be a demo of a pro game you downloaded.
Create VODs your casts and keep analysing what you could do better. NEVER try and be someone else in your cast, just be yourself. Look for criticism of your VODs from other casters or contemporaries rather than your friends or random people who can't give objective criticism.
Get better at it, and once you're decently satisfied with your work, approach smaller tourneys with offers to cast. I'm sure they'll not say no. There is no shortcut, you just have to slog your ass off studying teams, their playstyle, your casting style, etc.
There's too many things to get into in this post, but I hope this helped. Any specific questions you have just post it here, and I'd be happy to answer them.