give them little hints. For one keep your room REALLY clean, clean the house for no reason and be super nice to your parents. Like if they say "oh i need to clean the kitchen" say "NO I will" lol

and when you feel you are comfortable asking them make sure you have both of there attention and tell them I have been responisble lately etc and I have been looking at some dogs and I found the perfect one...etc

I hope that helped lol