What is the rhyme scheme of a sonnet?

It depends whether it’s a Shakespearean sonnet or a Petrarchan sonnet.

A sonnet is fourteen lines, but the different types rhyme them differently.

A Shakesperean sonnet, for example, has this rhyme scheme:

##”ABAB CDCD EFEF GG”##

A Petrarchan sonnet has this rhyme scheme:

##”ABBABBA CDECDE”##

The scheme for the second half can change.