Most important is to try to understand why your nails and hair have never grown well in the first place. Making sure you are getting proper nutrition and that you are digesting your food properly is very important. Without these components in place your body will not have the building blocks necessary to make strong collagen.
It is also very important to investigate your thyroid function. Brittle nails and hair are a hallmark sign of an under active thyroid so make sure to have your doctor check this at your next visit especially if it is coupled with other hypothyroid symptoms like weight gain, fatigue, low body temperature and depression.
Biotin can be quite helpful at improving hair and nail growth and strength. It is considered very safe so it might be worth a trial period of about 4 weeks to determine if it might be a useful supplement for you. Zinc and other minerals are also critically important for this process to occur correctly so you may consider adding a good multi-vitamin or multi-mineral supplement as well.