"I like a girl."
Translation:Jeg liker en jente.
The indefinite article follows the 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.