"The first girl's dress is the prettiest."
Translation:Rochia primei fete e cea mai frumoasă.
It is, you are right. However, when placing an adjective/ordinal numeral before the noun, the article is applied to that, instead of to the noun. Thus, you have "primei" instead of "prime" and "fete" instead of "fetei".
https://dexonline.ro/definitie/fat%C4%83/paradigma - this might be useful to you. You can search any word and click on "conjugări/declinări" to see declensions or conjugations.