A good salsa is useful as an appetizer dip, as a garnish on beans and meat, and as a sauce on tacos and enchiladas. This is a very good salsa to use on tacos and fish tacos.
As usual, I encourage you to start with this recipe and, in the future, modify it according to your own taste.
6 medium tomatoes, diced
2 cloves garlic, minced
1/2 onion, minced
3 tablespoons cilantro, chopped
2 jalapeño chiles, seeded and chopped
juice of one lime
salt and pepper
Just mix the ingredients, let the flavors meld for a while, and enjoy.
Some modifications you might consider: a few dashes of Tabasco, 2-3 tablespoons chopped green chiles, a pinch of oregano and/or marjoram. You can also experiment with additional lime juice, some olive oil, and jalapeños.
Have fun and enjoy!