Unfortunately, nonsurgical procedures (i.e., soft tissue fillers) for altering nostril shape do not effectively work. However, depending on the type of nostril asymmetry that exists, there may be less extensive surgical options such as alar base surgery that can significantly improve the appearance of problems such as unequal nostril size, alar flaring (the excessively rounded appearance of the nostril rims), or the nasal base being too wide. Other problems including crookedness of the nostrils or nostril rims that are asymmetric in height need more extensive surgery and are typically addressed during rhinoplasty surgery. You should consult with a board certified plastic surgery to discuss your concerns and see what options may be available to you.
If you have significant asymmetry and you are unhappy with the appearance, then you should discuss it with your parents to see if you can be evaluated by a plastic surgeon.
Unfortunately, nostril shape cannot safely be changed non-surgically. Most of the time, the deformity is imagined to be much worse by you than by others. I would recommend speaking with a professional about your feelings at least until you are older. If it still bothers you, then rhinoplasty will be for you. Good luck and be well.