The key to frizz-free, defined curls is adding moisture to your hair, preventing damage, and avoiding excess friction (like drying your hair with a rough towel).
Look for products designed to moisturize curly hair, such as nourishing conditioners and curl defining creams. When styling hair, apply styling products when your hair is wet. The additional moisture helps ensure that products are distributed evenly in your hair.
Whenever possible, avoid heat styling to prevent damaging hair. If you must use hot tools to create your perfect look, lower the heat setting and always apply heat protection before styling.