Pat,
Why canned tomatoes?
This is how I do a basic, fresh salsa (it's probably more of a pico-de-gallo, but people like it):
About 6-8 medium-size on-vine tomatoes.
(These have thick flesh & less seeds, etc.)
1 Large, sweet onion, diced
2 Anaheim peppers - seeded & diced
1 small can chopped green chiles, drained
2 large cloved garlic, minced (run thru press)
1 fresh lime
1 bunch of fresh cilantro - pick the tops off, to remove stems & chop coarsely
Salt
Pepper
I start by cutting the tomatoes into wedges, and then scrape the goop & seeds out. Then, I dice 'em.
Add all the other stuff in a non-reactive bowl (plastic, glass, or ceramic - the acid in the tomatoes can react with metal & introduce unwanted flavors)
halve & squeeze the lime directly into the bowl. Add a little salt & pepper to taste
Add some hotter chiles if you like
Let it sit for a few hours, covered in the refrigerator, to let the flavors mingle.
Allow to come to near room-temp before serving.