Salsa has too long been a bastion for the onion, but it's totally unnecessary. Unless you want your salsa to taste like armpit, toss out the onion and go back to the core elements that make a salsa great.
If you're really not into the DIY thing, there is one commercially available salsa with no onions, and it's excellent. I've posted a review here: Salsa With No Onions
What does salsa absolutely have to have for it to be valid? Chili peppers. You need other stuff too, but that is the only ingredient I'm calling non-negotiable. One of the things to think about before you begin is appearance. If you mix red ingredients and green, you end up with brown salsa; not that there's anything wrong with that, but if you are big on appearances, either pick on e or the other, or keep the chunks big. I'll put up a list of things I've found to be good salsa ingredients. Pick and choose what you like or have on hand. I'd recommend at least one item from each section:
Chilies (mandatory):
Habanero. Great flavor, but deadly hot (orange).
Fresno peppers: Excellent and quite hot (red).
Jalepeno: Boring, but easily available, hot (green).
Poblano and Anaheim: Mild, ...
