Are you making it from fresh tomatoes? That seems like it would be rather difficult unless you are making it when tomatoes are in peak season and are fully ripened. The typical tomatoes you see in the grocery store year round, off season, don't make a very good sauce (or soup) at all. They are too watery and bland, and you will likely wind up with a pink sludge instead of a deep rich red flavorful sauce/soup.
I would probably use some canned crushed tomatoes (San Marzano if you can) then add some broth and tomato paste. Perhaps start out by sauteing some finely chopped carrots, celery and onion until they are soft and translucent but not browned, then add in your crushed tomatoes, tomato paste and broth (chicken would be my choice, but a good vegetable broth would work too) until you get the desired consistency you want. Let it simmer for a while until it reduces a bit, then cool. Then blend it in your blender to get it really smooth. Put it back in a pot and heat it through again. If you want it creamier, add in some heavy cream. Some fresh herbs would be good too, such as basil, and finally season it to taste. You'll probably need a pinch of sugar too to cut some of the sourness from the tomatoes.