This is a popular and recurring topic here. If you use the Search feature, you'll find quite a bit of information.
What makes a good clam chowder, in my opinion, has a lot to do with where you grew up as there are many regional variations. A chowder that recalls memories of your youth is likely to be what you like best.
So for me, a good clam chowder has big clam chunks and is generous with them. And should be a bit light on the potatoes. Basically the opposite of a commercially produced or canned clam chowder where they're economizing.
The broth is key. It should have thickness, a clear clam taste and a richness from the dairy. These features are at odds with each other as the you need plenty of clam liquor/juice but that makes the broth thin besides requiring extra dairy. My preferred work around is to simmer the potatoes and vegies in clam liquor with a little white wine, infusing them with flavor and reducing the volume of the clam liquid while concentrating its flavor.
This next part is contentious. To thicken or not. Thickening tends to cloud the flavor but produces an excellent texture. If you've got a well flavored clam liquor reduced in cooking, I think thickening is a good technique. I use a roux, but Paul Prudhomme has a technique with potato cooked until falling apart to thicken and bring in more potato flavor.
Balancing salt is tricky as it's easy to be too salty. Just before serving where you correct seasoning with salt and pepper, squeeze in some fresh lemon juice and stir through. I find this accents the flavor well but doesn't require more salt to do so.
As to the clams, i use a 51 oz can of clams chopped fairly large. I get them at Costco. Fresh clams are available now and then but I usually use canned.
Do a search though and you'll find my as well as many other recipes.  | 
