ကၽြန္ေတာ္ is used in formal setting, as well as when you want to be more polite. so, it's safe to address yourself as ကၽြန္ေတာ္ when you are not sure how to address yourself. ငါ is for speaking with friends, people younger than you in informal setting. က်ဳပ္ is more commonly used in rural areas, some people used it when they are being sarcastic.