For nouns ending with a vowel (the majority) you only need to add an -s to form the plural.

a porta = the door (f; sing.) — as portas = the doors (f; pl.)
a mesa = the table (f; sing.) — as mesas = the tables (f; pl.)

o livro = the book (m; sing.) — os livros = the books (m; pl.)
o carro = the car (m; sing.) — os carros = the books (m; pl.)