You can start by letting them know why you want a dog for a pet. Let them know that you will home train it and take care of it. You can also tell them that it will be good for security purposes in case there are thieves around.
It's about negotiating. Besides telling your parents all the benefits of owning a dog, show them you are responsible enough for it by getting good grades, doing your chores without having to be nagged about it. Ask them: What do I have to do to get a dog? And do it!