1. Juan es estudioso. Le gusta leer. - This means Juan is bookish. He likes reading. Cantar means to sing, esquiar to ski, and bailar to dance.
2. Marisol es una chica simpatica. Since Marisol is a girl, you need the feminine form of the adjective simpatico, which is simpatica. And since there is only one of her, you also need its singular form (without -s).
3. The serious boy = el chico serio. Adjectives go after the noun, and since boy is masculine, you choose el, instead of la, which is used for girls.
4. A talented girl = una chica talentosa. You use una, instead of la, because una means a, and la means the.