In my early writing, all of my characters were exactly the same person. They all spoke the same, made the same types of jokes, reacted the same, etc. I think they were all just me in disguise.
To practice - write each and every day if possible - then try to attend professional writer's conferences where you can learn your craft, get to know fellow writers, and meet editors and agents.
Don't ever let the other stuff get in the way of your inherent skills as a kick-butt storyteller. Move the reader, make them happy and sad and excited and scared. Make them stare into space after they've put the book down, thinking about the tale that's become a part of them.
I really love middle-grade. Middle-grade books have a little more of a magical, light-hearted feel. You can be a little bit more quirky, you can have a little more humor. It doesn't get so dark and deep.
