In French when do you use l' and when do you use le or la?

In French, you use l' when the next Word that 'le' or 'la' is describing starts with a vowel, like in the Word 'aliment', which is a case where you would say l'aliment instead of le aliment You use 'le' or 'la' when it is before a Word that starts with a consonant, like the Word 'papier'. In that case, you would say le papier, and not l'papier.