I'm not a native speaker but here's what i understand of the genitive case endings:

In the singular:

-neuter nouns take the -ου ending

-ος becomes -ου

-ης/-ες become -η/-ε (masculine)

-η/-α become -ης/-ας (feminine)

In the plural: all nouns take the -ων ending

In the Tips & notes (found at the top left of the page) there is an explanation of the Genitive with examples.

For a real in-depth grammar review see here