Ok, so being a parent and a pet owner I have to point out a few things. This is not to be rude, but to help you think from a parents point of view, which if you can do that may help you get a dog or see your parents point.
1. Pure breed dogs cost a ton! They also have more problems. Like joint and hip problems you might want to research (could help you if you already have researched when your parents bring it up)
2. Dogs live a long time. Like 16 years. Not knowing how old you are I can only guess, high school maybe? That means when you go to collage who will be taking care of your dog? Renting with pets is hard and if makes finding a place harder and more expensive. How will you afford that?
3. Vet bills! Flea meds! As I said pure breeds often have more issues and it's expensive! My dog has a flea allergy, she always has to be on flea meds. If not she will bite herself raw. She's also allergic to bees. She almost died once when stung.... that was a vet bill!
4. Dog food! We get are animals good food and it costs a lot! But better food means healthier pets.
5. Walks and training. German sheaperds need lots of excersize! Look up what is recommended or the bread and really think about if you can do that every day! Also, they can be very aggressive! We had one growing up. He was great, but even after training and walks and everything he was extremely aggressive if he persived someone new as a threat. We had to talk to the breader and trainers. Some are just more aggressive and it can be very hard to brake that. As a young child I was bit by a German sheaperd who had irresponsible owners and still have scares from it, that can be a huge law suit. Just something to think about.
These are some of the things your parents will very likely bring up. I suggest you do lots of research, figure out what to do and how to pay for everything. Write down a plan. Think of it as an essay homework assignment. Make your argument and have all the questions answered before they can be asked. But remember this isn't something you can just stop playing with when your done, it's a living creature that will be around a long time, so think carefully about it. If you show you have seriously thought about it, you have a better shot.