My friends sometimes ask me for my salsa recipe. It's not really mine; it's (mostly) from a family friend (Auntie Angie). I don't really keep track of how much I put in of each ingredient, but I can give you ballpark figures. Let your taste buds be your guide.
4-6 cups coarsely chopped tomatoes
1/2 cup finely chopped red onion
1/2 cup finely chopped cilantro
juice of ~1 lime (taste as you go, and don't you dare use
lemon!)
1 jalapeno (fresh or pickled), seeded and minced
salt to taste (about 1/2 tsp?)
a tiny pinch of oregano (optional, definitely changes the
character of the salsa)
You can also make a fruit salsa, which is a nice change in the summer. The only difference is that you replace the tomatoes with a mixture of chopped fruits (melon for bulk, some strawberries and peach for flavor, a little bit of apple for texture, and whatever else you can find -- mango, pineapple...).