In my mind salsa is like guacamole. The best guacamole only has a few ingredients. Avocado, salt, and a little lemon juice. The key being fresh, ripe avocado.
Same with salsa. Tomatoes, onion, a little cilantro, a little hot pepper to make it as hot as you want, and salt. The key here is vine ripened tomatoes!
As a poster mentioned above, don't overdo it on the onion. For ratios, make some, taste it and add what you think is missing. It also tastes better after sitting a while in the fridge.