If "one" is a numeral, it is translated into Spanish as "uno", as numbers in Spanish have no gender.
It's different when you can replace it in English by "a" or "an", in this case we're talking of a determinant and it depends on the word coming after it, if it is a word with feminine gender you translate "una" (a house = una casa), if it is masculine you say "un" ( a man = un hombre; a bridge = un puente).
"One" can also be a pronoun, in this case it depends on the gender of the noun it is replacing ( I've got a house, but I want to buy a new one = tengo una casa, pero quiero comprar "una" nueva).
"Unos/unas" is the plural form, but they are both related to the English word "some".
In the song "One" is a numeral, "we are one, but not the same" the translation should be "uno" meaning "one unit".