It depends on the gender of the noun.
El aeropuerto = masculine singular
La escuela = feminine singular
Los libros = masculine plural
Las casas = feminine plural
You have to learn the gender of each noun. Don't just learn "dress = vestido." Learn "the dress = el vestido." Put the word for "the" on all your flash cards.
Studyspanish.com (Grammar Unit One) has guidelines that will make knowing which article to use a lot easier.