Welcome. You say you really want one, if you do end up getting piggies, please get at least two. Pigs are social animals and do best in either same sex groups, or if one of the genders is altered. In addition, please ADOPT future cavies from a rescue, shelter, or someone rehoming them. There are so many homeless animals out there.
Another note, if your parents say 'yes', are they prepared to pay for large vet bills, proper food, bedding, etc? Will they allow you to have a proper sized cage? Just some things to consider before getting a new pet.