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.
Down the road a bit, I would like to write a couple of stand-alone adult novels, especially in the horror genre. I've got lots of things up my sleeve.
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.