About this, when learning portuguese this is one of the hardest parts. Not always it'll work like:
"A woman" - woman is female so a should translates to uma.
In this case, a is in fact translated to uma but not because a woman (a real woman) is female, sometimes it works different.

For example:

"A house" or even "a city" as victor already used - how can you possibly tell if a house or a city is male or female (grammatically)? Common sense would tell you to go for um as proper translation but it's wrong. The correct translation is uma casa and uma cidade, not because we see them as female figures in any mean, but simply because it is like that in our grammar.

The best way to fix that in your mind is practicing by hearing/reading/talking, but there are some tips and cheats (though they wont work every time).
I'll drop this link (in portuguese) that victor provided me and ended up being very useful: Portuguese for foreigners - Nouns

Basically it explain how every noun has a gender (also a number and degree? Still not sure how to translate it) and not only the article but the whole sentence structure can change depending on that. Here's a small part of the text:

Primeiramente aprenderemos as flexões dos substantivos:
Gênero - Todo substantivo possui um gênero, que pode ser masculino ou feminino. Geralmente substantivos terminados em -a são femininos e substantivos terminados em -o, masculinos. Exemplo:
Braço (masculino)
Livro (masculino)
Calça (feminino)
Bolsa (feminino)

in name of curiosity: Braço = Arm/Livro = Book/Calça = Pants/ Bolsa = Bag
though some of these can change depending on context


Summing up, most of the time seeing if the noun ends in o or a makes a lot of difference to determine if it is male or female.

Any questions just ask me or victor