Nouns ending in - al change to - aux in the plural, eg:

• un journal (a newspaper) → des journaux (newspapers)

Nouns ending in -eu, -eau or -ou change to -eux, -eaux or -oux in the plural, eg:

• un jeu (game) → des jeux (games)
• un gâteau (cake) → des gâteaux (cakes)
• un bijou (jewel) → des bijoux (jewels)

Nouns that already end in -s,-x or -z in the singular form don't change in the plural, eg:

• un cas → des cas
• un choix → des choix
• un gaz → des gaz

Also watch out for some irregular plurals:

• un œil → des yeux
• Monsieur → Messieurs
• Madame → Mesdames
• Mademoiselle → Mesdemoiselles