If your parents don't think they are ready for another dog they probably aren't and personally i wouldn't push it. From personal experience, I can say that if you whine and bug them about it you'll get no where. Maybe let the subject go for awhile. In the mean time you can research breeds, when you know what kind of breed you want write down info about it and present it to your parents. This way they know about the dog you want to get and they know that you are being mature about it.