实话告诉你吧。(Shíhuà gàosu nǐ ba.)Let me tell you the truth.

Explanation

“实话告诉你吧”这个短句一般用作插入语，位于句首，作为告诉别人真相的提示语。也可以说成“实话跟你说了吧”。
“实话告诉你吧(Shíhuà gàosu nǐ ba.)” is usually used as parenthesis at the beginning of a sentence. It signifies that the speaker is going to tell the truth. We also say “实话跟你说了吧。(Shíhuà gēn nǐ shuō le ba.) ”for the same meaning.
Words:

真相zhēnxiànɡ:truth;
熟悉shúxī:familia.

Sentences

1、Shíhuà gàosu nǐ ba，tā zài piàn nǐ, tā gēnběn méiyǒu shēngbìng.
实话告诉你吧，他在骗你，他根本没有生病。
To tell the truth, he has lied to you; he is not ill at all.

2、Jiǎ:Nǐ zěnme nàme shúxī Běijing?
甲：你怎么那么熟悉北京？
A: Why are you so familiar with Beijing?

Yǐ: Shíhuà gàosu nǐ ba，wǒ jiùshi zài Běijing zhǎngdà de.
乙：实话告诉你吧，我就是在北京长大的。
B: To tell you the truth, I grew up in Beijing.