Your sunglasses must be bigger size than what you need. So, you shall buy a pair of smaller sunglasses the next time. Right now, what you can do is adjust your sunglasses frames so as to fit for your face. To avoid break your sunglasses frames, you'd better go to an glasses shop and had an optician adjust the sunglasses. And then check the sunglasses frames. If still not fit for you, adjust it again. So, be patient.
If your sunglasses often slide down your nose, it means that your sunglasses are too loose for you. Generally speaking, as long as the sunglasses can fit you properly, it won't slide down your nose. So you should take your sunglasses back to the place where you bought them and have them adjusted so that it can fit you properly, neither too loose nor too tight. Or you can get a new pair of sunglasses that are customized for you.
There must be something wrong with your glasses. The size is not suitable for you. In term of this problem, my suggestion is that you can adjust your sunglasses frames so as to make it fit your face. But in order to avoid breaking your sunglasses frames, you'd better go to an glasses store and ask a professional for help to adjust the sunglasses size. It will not take you a long time, for it is just an easy job for them. Next time you buy a pair of sunglasses, you should have a try to make sure it is suitable for you or not.
Have your parents take you back where you got them. They should be able to adjust them where they will stay up or explain why they will not. It could be a poor choice of frame (too big, poor fit on your nose, poor quality materials that don't keep adjustment). It could be that you or your parents passed on special features like high index lenses that would make your glasses lighter. There is technology out there that will make just about any prescription wearable without slipping.