college cook summed it up well.
i almost always add the liquid, either puree or juice. the only time i don't add it is if i have to make a sauce very quickly.
different tomato sauces can have different thickness and texture. some can be fresh and thin, others cooked a long time to deepen the flavor and thickness. and then there's every variant in between.
sometimes you want a chunky sauce, so i'd suggest using canned whole tomatoes, well crushed by hand as they are added to the pot, along with the accompanying liquid. if you want it thick, cook it a long time on very low heat, remembering to stir often.
if you need to thicken it up quickly, add a 1/2 can of tomato paste. it acts as a slurry would when making brown gravy. (a slurry is flour or cornstarch, diluted into cold water, then added to the pan drippings from a roast and another liquid like wine, water, or stock to make a gravy.)
sometimes you want a more even, consistent sauce, so finely chopping canned tomatoes or passing them through a food mill or sieve, either chopped or whole, will give you a more homogenized result. again, add the liquid and cook down until the thickness you desire. use the same trick with paste as i'd mentioned above to speed things up.
using fresh tomatoes is a slightly different ballgame, having to deal with variations in the tomatoes such as ripeness, amount of water, seeds, etc..
it's easier, i think, to start learning how to make sauce from scartch using good canned tomatoes. look for imported tomatoes, from san marzano, italy. if you can;t find those, look for oragnic brands such as muir glen.
A true lover of nature does not despair now that his mistress has turned a colder cheek.