。
She was wearing a white dress.
Literal
She [topic-は] white dress [object-を] was-wearing.
ドレス is the loanword for 'dress' — specifically formal wear, typically a Western-style gown, in Japanese. Everyday casual dresses are often called ワンピース (from 'one-piece'). 着る is the right verb here (a dress is a torso-covering garment).