Your dictionary may tell you that じつは means ‘actually’.
However, it means more like ‘to tell you the truth’.

せんせい
Let’s use 実は when you reveal something unknown yet.
Example
- 実は私がやったんです。(To tell you the truth, I did that.)
- 車をこわしたのは実は私です。(To tell you the truth, I broke the car.)
- この家には実は幽霊がでるんです。(To tell you the truth, there are ghosts in this house.)
- 実は私の父はアメリカ人なんです。(To tell you the truth, my father is American.)
- 実はこのあいだ宝くじに当たったんです。(To tell you the truth, I hit the jackpot a while ago.)