First, roughly chop up your onion, tomatoes, jalapeño (remove the seeds and membrane if you don't want it to be too spicy), and cilantro.
If your tomatoes seem pretty watery, you can cut them up, lightly salt them, and let them sit on a towel for 15 - 30 minutes. Pat them dry, then use for your salsa. The salt will draw out the excess water in the tomatoes and will keep your salsa from getting too watered down. (If you use this method, use less salt in your salsa!)
Next, add your tomatoes, jalapeño, most of your cilantro, and your onion to your blender. Give it a good, rough blend.
Stir everything together and scrape down the sides of your blender, then add the rest of your cilantro, your salt, and your cumin. (If you salted your tomatoes, use less salt now.)
Blend to your desired consistency and serve immediately.