Same problem. "I'm going to tell you" or "I need to tell you"?

I'd love to know the answer to this, too. I originally learned 要 as "need/want," so that's always the first way I interpret it, but I feel like I'm never really sure how to distinguish a genuine instance of wanting/needing to do something versus just being "going to do" something. "我要睡觉" -- "I need to sleep" or "I want to sleep" or just "I'm going to sleep"?