1. "J'ai les cheveux lisses." (pronounced: "zhay lay she-vuh lee-ss") This is the most common and literal translation. It focuses on the hair being straight.
2. "Mes cheveux sont raides." (pronounced: "may she-vuh sohn red") This emphasizes the hair being straight and stiff, rather than wavy or curly. It can be a bit more formal and less common.
Both are correct and understandable, but the first option is generally more natural and casual.