The verbs cocinar and preparar may be confused in some situations. The verb cocinar means to "cook", so it is never used with a direct object:
Esta noche voy a cocinar – Tonight I’m going to cook (incorrect *cocinar la cena)
Cocinar involves a thorough cooking process, being at the stove... You can say about your hobby:
Me gusta cocinar. Los fines de semana siempre cocino – I like cooking. At the weekends, I always cook
The verb preparar is used for food and intangible things:
Esta noche voy a preparar la cena – Tonight I’m going to cook / make dinner
Soy profesor, y por la noche preparo las clases – I'm a teacher and in the evenings I prepare my classes
"To prepare for" should be translated as prepararse (always with the preposition para):
Tengo que prepararme para sacarme el DELE (diploma internacional de español) – I need to prepare for the DELE exam