If you haven't already, start by chopping the vegetables. Finely dice the onion, tomatoes, avocado, and jalapeno peppers, and finely chop the cilantro.
1 lb Roma or plum tomatoes, de-seeded and finely diced, ¼ cup jalapeño peppers, finely diced, ½ cup fresh cilantro, chopped, 1 medium yellow onion, diced, 1 large ripe avocado