"It is a new shirt."
Translation:C'est une nouvelle chemise.
Nouvelle needs to come before chemise as it is describing the age of the shirt
Certain adjectives are placed before the noun, some which you can memorize with the acronym "BAGS":
-Good and bad
-Size (except for grand with people)
Keep in mind that neuf should be placed after the noun as it is a different meaning of new (it may be useful to think of neuf as 'brand new')
Hope that helps
It can indeed be "c'est une chemise neuve". The nuance is that it means "newly bought".
In the case of a shirt, both version would be similar in meaning, but if it were about a car, for ex, "une nouvelle voiture" and "une voiture neuve" are different: "neuve" means brand new (never used), but "nouvelle"' may mean a second-hand car, that is only "new" to you.