Starting with a one- or two- syllable adjective like pretty, you add suffixes to form a comparative prettier and a superlative prettiest. For words with more than two syllables, you instead add more or most, for example intelligent, more intelligent, most intelligent.

We normally use the comparative to compare two things or people:

Anika is prettier than her sister.

We normally use the superlative to identify the single most whatever in a group. Because we are talking about one specific member of the group, we use the definite article the.

Anika is the prettiest girl in her class

There is one exception when talking about the most whatever: when the group has only two members, we use the comparative rather than the superlative. We are still talking about one specific member of the group, so we still use the definite article:

Anika is the prettier of the two sisters