。
She made up her mind to become a doctor.
Literal
She [topic-は] doctor [to-に] become-let's [quotative-と] resolved.
The pattern [volitional + と + decision verb] frames an internal commitment: 医者になろう is the volitional 'let's become / I shall become,' と quotes that thought, and 決心した closes with 'resolved.' The whole construction is more emphatic than the more measured ~ことに決めた — 決心する carries the sense of bracing oneself for a long-term commitment, often used for career paths, lifestyle changes, or moral resolutions.