Le stylo (masculine singular) - This is the most common way to say "pen" in French. You would use this if you're talking about a pen in general or a specific pen.
Les stylos (masculine plural) - You would use this if you're talking about multiple pens.
For example:
* J'ai un stylo rouge. (I have a red pen.)
* Les stylos sont sur la table. (The pens are on the table.)
Let me know if you have any other French vocabulary questions!