"a coffee"

Translation: o cafea

[TheChairma2]

How do I know when it's "un" or "o" ?

[BrasovDesign]

You need to know (by learning them) the genders of the nouns.

In this case, cafea is feminine so the correct indefinite article/numeral is "o".

[potestasity]

The rules are a bit complex and there are several exceptions, but these rules here should cover the vast majority of words: https://en.wikipedia.org/wiki/Romanian_nouns#Gender_assignment:_phonetic

[IXL_hater]

Is it me, or does 'cafea sound like something else?