Hi Anna Liisa,
Just to add to what Chris said-
nouveau/nouvel/nouvelle (new) is normally placed before the noun as in:
Mon nouvel ami s'appelle Martin = My new friend is called Martin
Ma nouvelle voiture est bleue = My new car is a blue one
Mon nouveau pote s'appelle Jacques = My new mate is called Jacques
but
neuf/neuve ( brand new) is always placed after the noun.
J'ai un vélo tout neuf = I have a brand new bike
Ma voiture est neuve = My car is brand new
But if you say:
"J'ai une nouvelle voiture", we don't know whether it is brand new or not, it is just 'new to you'
Hope this helps!