You can use 반말 possibly to a VERY close family members including parents and grandparents and friends. 요 to make the phrase more respectful. You have to use 존댓말, (for example) when you talk to older people, teacher, boss, professors or not-so-close older family members.

So , in general, 반말: in family, to younger people, between VERY close friends even if they're older than us. -요: between strangers, colleagues, to friends older than us (so not very close) -ㅂ니다 : to people of a certain elite like grandfathers, grandmothers, professors, ambassadors etc..