How do I form the superlative (e.g., "biggest") of adjectives?
For predicative/adverbial use, add am ...-sten. For attributive use (before a noun), add -st to the adjective, then decline it. Some adjectives take an umlaut.
Example
Predicative/Adverbial: Das ist am schnellsten. (That is the fastest.) Er fährt am schnellsten. (He drives fastest.)
Attributive: Das schnell*ste Auto* (the fastest car - declined weak)
alt → am ältesten / der/die/das älteste
gut → am besten / der/die/das beste (irregular)
groß → am größten / der/die/das größte (irregular, adds -ß instead of -ss)