Here is a chart showing how to construct the superlative form in English:

One Syllable Adjectives

Place "the" before the adjective and add "-est" to end of the adjective (Note: double the final consonant if preceded by a vowel).

Example: cheap - the cheapest / hot - the hottest / high - the highest

Example Sentences

Today is the hottest day of the summer. 
This book is the cheapest I can find.

Two, Three or More Syllable Adjectives

Place "the most" before the adjective.

Example: interesting - the most interesting / difficult - the most difficult

Example Sentences:

London is the most expensive city in England. 
That is the most beautiful painting here.

Two Syllable Adjectives Ending in "-y" Place "the" before the adjective and remove the "y" from the adjective and add "iest".

Example: happy - the happiest / funny - the funniest

Example Sentences

New York is the noisiest city in the USA. 
He is the most important person I know.

IMPORTANT EXCEPTIONS

There are some important exceptions to these rules. Here are two of the most important exceptions:

good

§ good - adjective

§ the best - superlative

Example Sentences

Peter is the best golf player in the school. 
This is the best school in the city.

bad

§ bad - adjective

§ the worst - superlative

Example Sentences

Jane is the worst student in the class. 
This is the worst day of my life.