We don't really say "知らせて". We only say that when we want someone to inform us. EX) 電話が来たら知らせて。 Let me know when someone calls me. You can say "教えて" instead of saying "知らせて" in almost any situations. So it's better to use "教えて" when you don't know which one is appropriate.

Hello~ 教えて-teach me 知らせて-inform me