1 large can whole tomatoes (29 oz.)
1/2 sweet onion
5-8 garlic cloves
1/2 to 1 jalapeño, seeded and ribs removed
1/2 to 1 (depending on size) bunch cilantro
1 tsp. salt or to taste
Rough chop all ingredients and then purée in a blender or food processor until fairly smooth. (You will probably want to go ahead and mince the jalapeño fairly fine, especially if you are using a food processor, just to make sure you don't leave any large chunks.)
Just a couple of additional tips on handling jalapeños if you have not done it before: do handle them with care. They are kind of deceptive little dudes that make you think you are just chopping a small green pepper until 20 minutes after you're done chopping them and your fingers are on fire. Gloves are really recommended when chopping jalapeños, but if you don't have any, make sure you wash your hands immediately and thoroughly after handling them, and while you still have the juice on your hands, be VERY careful not to do anything like rubbing your eyes or touching someone else. And just in case you are very unfamiliar with jalapeños, the seeds and the little whitish ribs on the inside are the hottest parts, so be sure to remove them if you don't want a whole lot of heat.
And there you have it!