However, depending on the context, you could also say:
* J'ai un stylo. (I have a pen.) - This is the most common and direct way to say it.
* Tu as un stylo? (Do you have a pen?) - This is a question asking if someone has a pen.
* Pouvez-vous me donner un stylo? (Could you give me a pen?) - This is a polite request for a pen.
Remember that the French language has a lot of variations depending on the situation and the tone you want to convey.