"It is a girl."
Translation:Det er en jente.
Before a noun is introduced, the demonstrative pronoun used is always det.
Det er en bok. Den er interessant. (It is a book. It's interesting.)
Det er et insekt. Det er farlig. (It is an insect. It's dangerous.)
When you want to refer to that noun, you have to change the demonstrative pronoun according to its gender (den - masculine/feminine, det - neuter). However, this is used only to refer to animals and things, not human beings.
Both words translate to "that/it".
En/ei pike is synonymous with en/ei jente but the former is rarely used and is also considered old-fashioned.