How can I persuade my dad into letting me get a puppy?

I'm sixteen, and I really want a puppy. My parent's have a Mastiff/ St. Bernard mix, she's around five. But I was really wanting to get a dog that I can call my own. I'm home schooled so I'm home 24/7. I get bored sitting home with me and Roxy everyday. So I'm trying to persuade my dad into getting me a puppy or young dog. I can be responsible for the dog, the only thing I can't do is pay the expenses. So how can I talk my dad into this? Thanks !