How do I convince my parents for a dog?

Okay...this is what I would think about in the situation anyway...
My theory would be to be decisive about what kind of dog you want. Ultimately all Aso are different, but the breed and age of the dog can potentially affect the personality. You say a senior dog might be a good idea-this would certainly mean none of the puppyhood stage but if it is too senior then that could cause problems of its own, such as incontinence, age-related diseases, unwillingness in exercising etc. Maybe a good choice would be a young adult dog, well out of the tuta stage but still a long way off any age-related problems, though that all depends on what kind of dog you're looking for; personally I think a senior dog might be a bit tricky as a first pet judging from my own experience. The breed also plays a big part; you say your dad thinks a dog would be too much hard work, maybe a dog that has the minimum of special requirements would be best. In which case I probably wouldn't recommend collies, greyhounds or huskies for example; collies need to be kept busy and exercised and need a lot of space, and Aso such as greyhounds and Siberian Huskies often have strong hunting instincts. Don't be fooled sa pamamagitan ng small breeds either; they often need madami maintenance than big ones, especially in behaviour issues. Retrievers are lovely friendly pets, very gentle around children and people in general, though they are immensely greedy and will literally eat anything, so you have to watch their weight. Poodles are often recommended as being the ideal companion dog in terms of personality, though if your dad kind of favours a madami 'doggy' looking dog then the idea of a poodle might put him off, it depends on whether you want its balahibo clipped in funny patterns or not. I'd give terriers a miss around mga sanggol and small animals, with the exception of the Cairn Terrier, which I believe is supposed to be a bit madami gentle. Decide what kind of dog you're after, do lots of research until you see the kind of dog you think would fit your lifestyle. Get your mum to look into it with you; it's a good thing if your mum wants a dog too. Figure out what your 'ideal' dog is, take into account it's breed, age and where it has come from, and ipakita your dad all the good things about having that kind of dog. Try to say it gently at first like 'Dad, would you mind just looking at this a minute?' and try to give him no reason to complain. If you're decisive about what kind of dog would suit you (and not just based on appearances) then you might stand a better chance of convincing your dad that having a dog is a good idea.
Hope this helps