"We have a child."
Translation:Vi har et barn.
It depends on the grammatical gender of the word after it. In this case, "barn" is neuter so you use "et". If the word had been masculine (e.g "hund" (dog)) you would say "en", and if it was feminine (e.g "jente" (girl)) you could use "ei" (but "en" is often also accepted).
If you're not familiar with grammatical genders, then this is just something you have to practice and after a while it will come naturally. If not, then anyone in Norway are going to understand you fine, it will just sound a little funny sometimes when you use the wrong article.