。
She became a singer.
Literal
She [topic-は] singer [as-に] became.
Nになる is the standard 'become N' construction in Japanese — N marked with に + the verb なる ('become'). A few siblings to know: i-adjectives become ~く+なる (寒くなる, 'become cold'), na-adjectives ~に+なる (元気になる, 'become well'). The に here is essentially the 'into' marker of state change. The verb 歌手になる is one of the most stable career-trajectory expressions in Japanese: 'become a singer.'