。
She started singing.
Literal
She [topic-は] sing-began.
歌い始める is a textbook example of Japanese's prolific compound-verb system: verb-stem 歌い (from 歌う, 'sing') + the auxiliary verb 始める ('begin') = 'begin singing.' Many Japanese verbs of action stack with ~始める this way (食べ始める, 走り始める, 雨が降り始める). Compared to using a separate verb of starting plus a connector (like '~のを始める'), the compound form is tighter, more idiomatic, and the default in everyday Japanese.