Dice the onion and tomatoes (I pull the seeds out of mine). Put them in an air-tight container. Quarter the lime and squeeze all of it over the onion & tomatoes. I use about 1/2 a bunch of cilantro, but chop it up pretty small and throw it in with the onion, tomatoes and lime juice. Half the jalapeno and slice one have into this strips. Then cut the strips into tiny squares. I use about half of a jalapeno (minus the seeds) in my pico, but you can use as much (or as little) as you'd like! Add to the other ingredients, stir and refrigerate.