Formal:
* J'ai votre crayon. (This is the most formal and literal translation.)
Informal:
* J'ai ton crayon. (This is more informal and uses the familiar "tu" form of "you".)
* J'ai le crayon que tu as oublié. (This is a bit longer but more specific, saying "I have the pencil that you forgot.")
Adding context:
You can also add context by saying:
* J'ai trouvé ton crayon. (I found your pencil.)
* J'ai récupéré ton crayon. (I picked up your pencil.)
* Je l'ai gardé pour toi. (I kept it for you.)
Choose the option that best fits the situation and your relationship with the person you are talking to.