Chop the onion into 1/4" or smaller pieces, add to mixing bowl.
Chop the tomato into 1/4" or smaller pieces, add to mixing bowl.
Finely mince the jalapeno, add to mixing bowl.
Smash then finely mince the garlic, add to mixing bowl.
If desired, remove stems from cilantro, then roughly chop. Some people find the stems too bitter. You can ignore that step if you don't object to the stems.
Juice the lime over the vegetables. Scrape the pulp if you want to.
Salt the mixture.
Stir it all together and let sit or refrigerate for 15-30 mins to let the flavors blend.