"Carro" is masculine, and needs a masculine article.
"Bicicleta" is feminine, and needs a feminine article.
There are exceptions, but usually word ending with "o" are masculine.
(but not all the masculine words are with "o", and not all the words with "o" are masculine)
And same for words in "a" being feminine (but there are exceptions, not all the feminine words... etc.. same than above).