Hair is hygroscopic which means it absorbs moisture from the air. So, you need to use a good serum to lock your cuticles that avoids your hair from soaking in excess moisture in the air."
For curly and dry hair: After shampooing your hair, apply a leave-in conditioner that is mixed with a drop of thick creamy serum. This acts like a coat, preventing the moisture in the air from being absorbed and your hair remain frizz free.
For oily hair/scalp: Use a good shampoo formulated for oily scalps. Gently massage your scalp for two to three minutes while applying shampoo, then leave it untouched for few minutes and rinse off till your hair feels squeaky clean.
A good conditioner is a must. If you have fine hair, apply a spray-on serum so that it coats the hair and avoids it from absorbing moisture in the air. Wash your hair atleast every alternate day.
For normal hair: You should also also follow this three step hair care regime - shampooing, conditioning, and using a good styling or hair care product.