The plural of nouns ending with -ão is formed by:

1. changing the -ão ending to -ões. This rule encompasses the majority of nouns ending with -ão.
As a rule of thumb (and/or when in doubt), change singular nouns ending with -ão to -ões to form the plural:
a lição = the lesson (f, sing.) — as lições = the lessons (f, pl.)
o tubarão = the shark (m, sing.) — os tubarões = the sharks (m, pl.)

2. changing the -ão ending to -ãos.
o irmão = the brother (m, sing.) — os irmãos = the brothers (m, pl.)

3. changing the -ão ending to -ães. There are only very few nouns where this is applicable.
o cão = the dog (m, sing.) — os cães = the dogs (m, pl.)