"I like a girl."
Translation:Jeg liker en jente.
The indefinite article follows the grammatical gender of the noun:
en for masculine nouns
ei for feminine nouns
et for neuter nouns
*Feminine nouns may be declined as if they were masculine, so you can opt for en with feminine nouns as well. How common this is depends on the noun in question, degree of formality, and the dialect of the writer/speaker.