Oui, je l'ai fait.
Here's a breakdown of the sentence:
* Oui: Yes
* Je: I
* l'ai: have (past participle of "avoir", meaning "to have")
* fait: done
This sentence is a direct translation of "yes, I have done it."
You can also use other variations depending on the context, such as:
* Oui, c'est moi qui l'ai fait. (Yes, it is I who did it.)
* Oui, je l'ai effectivement fait. (Yes, I did it indeed.)
* Oui, je l'ai bien fait. (Yes, I did it well.)
The best option depends on the situation and the level of formality you are using.