This is a difference between Portuguese and English and it's actually pretty complex. But put simply, when nouns start a sentence they need to have the correct article before them. If these nouns are people's names, we just remove the "extra" articles in English but Portuguese keeps them.
When it's a different type of noun, "shirt" for example, then both English and Portuguese keep the article. I guess you could say Portuguese is a bit more consistent in this situation than English.
A Luciana é de Portugal. - "Luciana is from Portugal."
O João está doente hoje. - "João is sick today."
A cadeira é azul. - "The chair is blue."
As casas estão em construção. - "The houses are under construction."