Articles must agree with the gender and number of the noun they accompany.

Definite Articles — The

• el — masculine singular: el libro
• la — feminine singular: la mesa
• los — masculine plural: los libros
• las — feminine plural: las mesas

Indefinite Articles — A / An / Some

• un — masculine singular: un perro
• una — feminine singular: una gata
• unos — masculine plural: unos libros
• unas — feminine plural: unas flores

Special Cases

• Feminine nouns starting with stressed A- use el in singular: el agua (but las aguas)
• After ser with unmodified professions, omit the article: Soy médico.

Spanish uses articles more than English — learn which contexts require them.

Reference:

Wikipedia: Spanish Articles

https://en.wikipedia.org/wiki/Spanish_language