Use _er, _est if the adjective has one or two syllables, e.g. happy, happier, happiest; high, higher, highest.
Positive degree: high
Comparative degree: higher
Superlative degree; highest
Use more or most if the adjective has more than two syllables, e.g. beautiful, more beautiful, most beautiful.
There are also some words that have special comparative forms, e.g. good, better, best; bad, worse, worst.