The superlative is used to describe the upper or lower bound of a quantity, e.g. the highest and the lowest, the best and the worst. In French, there are 5 ways of using the superlative that you need to learn: when using it with adverbs; with adjectives; with nous; with verbs; and of course, the irregular ones.
First of all, you must know these two words, the least and the most: le moins and le plus
1+2) Superlatives with adverbs/adjectives - It's the same structure for both. You put 'le plus/le moins' followed by the adverb/adjective. REMEMBER that with adjectives, the definitive article has to agree with the gender and number with the noun they are referring to. E.g. C'est l'athlète le plus rapide du monde (They're the fastest athlete in the world)
Superlatives with nouns - You put 'le plus/le moins' followed by 'de' and then the noun. E.g. Elle a le plus de jouets de tous ses frères et soeurs (She has the most toys of all her siblings)
Superlatives with verbs - The verb is followed by 'le plus/le moins'. E.g. C'est le fils de Jean qui joue le plus dans le club (It's John's son that plays the most in the club)
Irregular superlatives - To express the best and the worst, French has 2 different forms for each. Le mieux (bien), le/la/les meilleur(e)(s) (bon[ne][s]), le pire (mal) and le/la/les pire(s) (mauvais[e][s]). REMEMBER bon(ne)(s) and pire(s) are adjectives so they need to agree with noun they are referring to. E.g. Il est le meilleur joueur de rugby du monde (He is the best rugby player in the world), c'est la pire chose qui m'ait arrivé (This is the worst thing that's happened to me).