Every substantive ist either masculin or femenin. The masculin substantives use the article "el" and the femenin the article "la":

Masculin Femenin
El coche La caja

The  are other substantives which can be used in masculin or in femenin:

Masculin Femenin
El niño La niña
El gato La gata
El profesor La profesora

As an informal rule, we could say that every substantive which ends in "a" is femenin and every substantive which ends in "o" is masculin. There are of course exceptions (el sofa, la moto) but you can normally apply the rule with high probability of sucsess.

The plural

Building the plural from spanish words is very easy because you only have to add "-s" or "-es" to the end of the word depending if it ends with vowel or with consonant. The articles for plural change to "los" for masculin and "las" for femenin:

Singular Plural
el coche los coches
la caja las cajas
el gato los gatos
el profesor los profesores
la profesora las profesoras
el ventilador los ventiladores
la televisión los televisiones
la raíz las raices

The last word (raíz) changed the "z" to "c". Words which end in "z" loose the letter and change it to "c" before the addition of "es".

The undefined article

We use "un" and "una" for the undefined singular substantives and "unos" and "unas" for the undefined plural substantives:

Singular Plural
Masculin un coche unos coches
un gato unos gatos
Femenin una profesora unas profesoras
una caja unas cajas

Vocabulary used in the examples:

caja - box
coche - car
gato,gata - cat
moto - motorbike
niño - boy
niña - girl
profesor,profesora - teacher
sofa - sofa
television - television
ventilador - ventilator