I will give you some tips:
-Trim it every 6-9 months only a little.
-Eat foods rich in protein, fatty acids, vitamin C, Biotin, B vitamins, Vitamin E, iron, zinc and silica. Examples of healthy food which are great for hair growth: salmon, meat, walnuts, spinach, eggs, sweet potatoes, lentils, yogurt, blueberries, brown rice, olives and chicken.
-Don't dye it. If your hair are already dyed, then you should ONLY dye the roots. Do so every 2-3 months.
-Avoid heat and start air-drying your hair.
-Don't wash it every day. Instead wash it every 2-3 days. If your hair is too oily, you can use baby powder or dry shampoo. It's very cheap.
-Use a sulfate-free shampoo.
-Don't play with your hair too much.
-Use a wide-toothed comb instead of a brush.
-Take Biotin supplements.
-Do hair masks by mixing avocado, coconut or olive oil with avocado (avocado is extremely healthy for hair).
-Drink plenty of water.
-Don't rip through your hair when you comb it.
I hope you will find my tips helpful enough :3