This is the 326th most frequent Ukrainian word.
почала
"Почала" means "started" or "began" (feminine singular, past tense).
Again, 'почала' refers to the initiation of an action, specifically for a feminine subject, in this case 'моя сестра' (my sister). It demonstrates the subject actively starting an ongoing or habitual action.
Моя сестра почала займатися спортом.
My sister began practicing sports.
In this question, 'почала' is used to inquire about the beginning of an action ('працювати' - 'to work') performed by the feminine addressee ('ти' - 'you') in the past.
Коли ти почала працювати тут?
When did you start working here?
The word 'почала' is used here as the feminine singular past tense of the verb 'почати' (to begin, to start), indicating that the subject, 'вона' (she), initiated the activity of reading.