Both phrases mean someone is telling something to someone else. However, there are hidden intricasies. 我跟你说 , means "I with you speak", so in otherwords, you're speaking to someone and you're going to tell them something. This will most probably be something of very little concequence- "我跟你说, 我很喜歡吃飯。", "我跟你说, 我覺得那個男生很帥" 告诉 means to tell something to someone that they dont already know, usually of much heavier concequence and suggests its something unusual that the other doesn't know- "我告诉你 , 這本書很好用" (you dont know this book is useful, so it informs you that it is), "我告诉你, 我懷孕了" (you dont know that person is pregnant, so this informs you). YOU CAN use both in the same circumstances, but I think I'd use these in this way.

They both mean that I will tell you something. But there are little differences between this two expressions. For example. When you don't familar with something, I will explain it for you so I can say " hai，我告诉你吧". When people want to tell others something unusual or interesting, they will say: 我跟你说，I came across a pertty handsome guy!

意思一样。 语气上，“我告诉你”稍微强硬一点点。带有一点“我知道的多，你知道的少，我是对的，你是错的，我来给你上一课”的感觉。但只是很少的一点点。 “我跟你说”更平等，更柔和一点。 只是极细微的差别。大多数时候可以不考虑的。

well...actually,there are difference between two phrases. 1. 我跟你说，this phrase,it is mainly colloquial one which is used in daily life. But you can not use it when you talk with the elder or older people,or the guys with higher status than you. If you do that, it goes to show you are a rude person. You can use it when you talk with the guys of same age or same social status,or guys who are younger than you or your subordinates. 2. 我告诉，well, this phrase, somehow, it is much powerful,aggresive with strong personal feeling.It can never be used when you are talking with guys of higher status or the elders.(f.g. you can say it when you are talking to your boss.) Besides, its tones are various and bring different consequences(bad or good), So, be careful and avoid unnecessary misunderstanding.When you are spaeking Chinese, bear its rules in your mind. Hope my answer will fit to solve your qproblem. Good luck.

Sorry !! here is a mistyping 我告诉你 ------ I tell " U "something . Ex:我告訴你我要去看藍球賽！