E.g. Marie est la fille la plus intelligente de la classe (Marie is the smartest girl in the class)

Two things to pay attention to in complex examples like this are:

1. la fille usually appears before la plus whereas in English it's the reverse order. Also see that 'la' appears twice.
2. de la/du/des/ de l' ('of the') always follows the superlative, even when in English we might say 'in'.

Note: you can also say:

Marie est la plus intelligente fille de la classe (Marie is the smartest girl in the class)

See also the related Le, la, les plus and le, la, les moins = the most and the least (superlatives of adjectives)

Here are other Superlative structures:
Le plus and le moins = the most and the least (superlative of adverbs)
Meilleur, mieux, pire / plus mauvais, plus mal = better, best, worse and worst (irregular comparatives and superlatives)

And Comparative structures:
Plus... plus..., moins... moins... = the more...the more..., the less...the less... (comparisons with phrases)
Better and better, worse and worse = de mieux en mieux, de pire en pire (comparisons)
De plus en plus and de moins en moins = more and more and less and less (comparisons with adjectives, adverbs, verbs)
De plus en plus de and de moins en moins de = more and more and less and less (comparisons of nouns)
Making comparisons with adjectives: plus... que, aussi... que, moins... que
Making comparisons with adverbs: plus... que, aussi... que, moins... que
Making comparisons with verbs: plus que, autant que, moins que
Making comparisons with nouns: plus de... que, moins de... que, autant de... que