Russian is not a particularly pro-drop language, even compared to many other Slavic languages. So by default, on its own, you should use the appropriate pronoun, even if it could be grammatically inferred.
But with enough context, when subject is already introduced, or in informal speech, the pronoun may be dropped. Sometimes it's even customary, e.g.:
Говорила же тебе: учись!
But this may be true even in English, which is considered formally non-pro-drop, e.g.
Like it! (meaning: I like it!, rather than the formal imperative meaning 'You better like it, or else...').
Note that in your example the verb suggests the gender, but not the person, thus dropping the pronoun leaves it unclear whether она, я, or ты говорила.
When all the person, number and gender (if applicable) can be inferred, it is normal, but still optional, to drop the pronoun:
Хочешь пить? = Ты хочешь пить?
Basically, when in doubt, say the pronoun, until you learn to feel it intuitively.