It's already been a few times, but hair growth is a process. While keratin is a protein that makes up your hair, you aren't likely deficient in it. There are vitamins out there that can help, but hair health is determined by 3 factors; Genetics, environment and lifestyle. Those are not in order of precedence as it's been said, “Genetics load the gun but lifestyle pulls the trigger.” Starting with overall healthy lifestyle for an internal approach is key. Eat plenty of fibrous veggies, quality protein(both local when possible) and keep the junk food in moderation.
Be active and outside as much as possible. Overall hygiene is important as a clean scalp is best for growing hair. Look for a sulfate free, plant based shampoo. (L'occitane, Phyto, and Bumble & Bumble are great) Wash your hair everyday and especially after working up a sweat as DHT build up on your scalp will promote hair loss. The idea that you shouldn't wash your hair every day comes from using low quality products.
Once you have the basics covered then you can look at things that will build off of them. Massaging your scalp or using a quality brush helps get blood their which will feed the follicles allowing them to grow healthier hair faster (think of a farmer watering his crops) 5-10 mins every other day should do it. Reducing stress is important. you've probably heard of cortisol (the stress hormone) as it's been buzzing around social media for a while. If you feel stress is out of control it will tax the systems of your body in many ways and hair loss is a common result. Look to activities that you enjoy and help you feel peaceful.
Building off of that you can look at treatments and vitamins designed to help. Kerotin Hair Growth Vitamins for Natural Longer, Stronger, Healthier Hair has shown to be very successful in helping people grow thicker healthier hair and combating hair loss. Its an all natural blend of vitamins, minerals and herb designed to nourish your scalp have the healthiest follicles (Back to the farmer analogy. Think watering fertile topsoil instead of a more sandy landscape) There are drugs out there that will help, but remember that they are foreign to the inner working of your body and if you are going to use them, they should be used as a short term band aid in conjunction with the above mentioned protocol to start the hair growing process at its roots (pun intended.) Hope this helps!