Combine chopped tomato, onion or red onion, a bit of garlic if you like, a hot chile pepper or two depending on how spicy you like it, salt to taste, and chopped fresh cilantro if you like it. You can splash on a bit of fresh lime juice or red wine vinegar, if you're so inclined.
How much of each? Well that is what makes salsa fresca so fabulous—it really depends on how you like it and what you have on hand. I find 4 tomatoes, a half an onion, a small clove of garlic, 1 chile, and a handful of cilantro suits me and my taste, so you can start there, play around with it, and find your own favorite combination.