if your an only child or if you dont have any friends then you can say that you are lonley and youd like some company.
if that doesnt apply to you then you can go for the BEG BEG BEGGING route.
or you could get an adult to try and convince them
you could say that it would teach you hoow to love and nuture something living.
plus you can list all the advantages and make them sound really beneficial then list the disadvantages and make sure that there is only a few and when you say them say them like their only minor issues that dont even matter.
good luck
ps. your parents wouldnt be mean they know you better than anyone they knnow if your a determained person that would care for the pet and not starve it or ignore it.
if they dont let you have it its probs cos they dont think your responsible to handle such a thing
dont give them a hard time about it after all their grownups their wise their cool and most important they know whats best.

Good Luck :