Informal:
* ¡JAJA, es broma! - This is the most common and informal way to say it, directly translating to "HAHA, it's a joke!"
* ¡Te estoy vacilando! - This means "I'm teasing you!" and implies a playful joke.
* ¡No te lo tomes en serio! - This translates to "Don't take it seriously!" and emphasizes that you're not being serious.
More formal:
* No te preocupes, era una broma. - This translates to "Don't worry, it was a joke." This is a slightly more polite and less playful way to say it.
* ¡No lo dije en serio! - This translates to "I didn't mean it!" and is good for when you want to emphasize that your words were not intended to be taken seriously.
You can also use emojis like 😅 (smiling face with sweat) or 😂 (face with tears of joy) to further emphasize that you're joking.
Ultimately, the best way to say it depends on your relationship with the person you're talking to and the situation.