Chop the jalapeno pepper: slice the pepper in half and taste one of the seeds for heat. If the pepper isn't too hot, finely mince it. If it's very hot, remove the seeds and membrane, and then mince it.
Combine all the ingredients in a bow. Taste and adjust the salt, lime juice, and cilantro.
Store leftover pico de gallo covered in the refrigerator for a few days, though best when fresh.