Normally a Portuguese word will be associated with its article. The article will agree in gender with its noun. The definite article (like "the" in English) in Portuguese is "o" for masculine and "a" for feminine gender in singular form. It is better to remember each noun and its article together as one unit.
Let's see some examples:

o gato	The cat (masculine singular)
o homem	The man (masculine singular)
a caneta	The pen (feminine singular)
a trovoada	The thunder (feminine singular)