A traditional Mexican Pico de Gallo is very simple, and only has few ingredients:
2 medium tomatoes (1/4 to half inch pieces)
1 jalepeno - I prefer a fine dice, which keeps the chiles from overpowering each bite
1/4 to 1/3 cup of cilantro, including at least part of the stem (I use about 1/2 cup)
1/2 cup diced onions - medium to fine dice
Salt the tomatoes to taste, then mix all the ingredients in a bowl together.
Add fresh lime juice, vinegar, or lemon juice (a couple tablespoons) and stir. The acid is important for the flavor, and it has preserving properties.
Really, that's it! You can add other seasonings or spices for a more unique flavor - chili powders, cumin, Mexican Oregano and fresh or granulated garlic make nice additions.
You can also play with the recipe to create your own unique salsa. I have subbed mango and pineapple for the tomato, which makes a very nice fruit salsa.