How do I know which words have "el" or "la" as their article?

"El" is the masculine article: El foco, El chico, El jardín. (Almost) all words ending in "o" will be masculine. Any other ending apart from "a" will also surely be masculine.

"La" is the feminine article: La cama, La piscina, La tarta. Any word ending in "a" will be feminine.