Hmmmm ... to me this would be a hard one, because the Mexican restaurants I go to vary a LOT in their salsas. I consider "cooked" salsas to be something I wouldn't bother trying to make myself, there are so many available in the stores. So when I make a salsa, I go for what I call a "fresh" salsa, or something akin to pico de gallo (please correct my spelling). When I do that, I take one or two really ripe tomatoes and chop, finely chop a small or half a large onion (the sweeter the better, but make do, and a couple of green onions will substitute), cilantro to taste (I don't think it tastes right without, but if you hate the stuff, and many do, just eliminate it or put in regular parsley). One clove of garlic, smashed and finely chopped. The juice from half a lime.
The heat is something you cannot regulate or recipe. In years of cooking with and growing peppers I've learned that even two peppers picked from the same bush at different times of the year can vary extremely in heat ... AND what I'd consider mild, a friend would consider incendiary and another would think wimpy. So when I do this, I do it for our (that is to say, husband and me) taste. Because I'm going for a "fresh" salsa rather than a cooked one, I buy (or pick from my garden, depends on the time of year) a pepper or two, then add it a little at a time until I have the heat we like (want to make your lips dance, but not ruin your taste buds for the rest of the meal). If you have a jarred version of peppers you happen to like the flavor and heat factor, use them, as much or little as you like.
That's just the basics; if you have a bell pepper, some celery, etc, hanging around the fridge, chop fine and add.
If you have a specific restaurant whose salsa you love, consider asking them if they would sell you some to take home. I've been eating Mexican food since I was a child in restaurants from coast to coast and border to border, and no two (well, aside from chains, which I avoid; I look for where the local Mexicans go to eat) have exactly the same salsa.