I prefer a light medium bodied white wine.
get a shallow cook top safe heavy pan - my favorite is by la creuset.
Take a stock pot of about 1 gallon, start by making a fine dice of equal parts celery, onion and carrot [1 cup of each], about a handful of salt, add a bouquet garni wrapped in cooking gauze of a couple sprigs each thyme and parsley, a pair of bay leaves, and a teaspoon black peppercorns roughly smashed. Chuck in 1 cup of white wine, and about 3 quarts of water. Simmer for about half an hour then strain out the solids. Chill it or use it right away. [I do know someone who wraps everything in gauze to make it easier to deal with. Up to you.]
Small fishies can be started in hot bouillon, larger pieces of fish you need to start in cold bouillon.
Since I prefer to do large fillets [like 2 servings per fillet] I take a heavy skillet, put it on the larger burner on my stovetop, lay the fillet or fillets in and add the bouillon to just cover the fillet. Then I turn the burner on to medium low - you do not want to boil, but to just barely simmer. You want to see a sort of shimmer in the pot. You cook it very gently until the flesh is opaque and barely starting to flake.
It is a great way to cook delicate fish like a trout or sole/flounder that also gives it nice flavor and is ultra low fat. If you do not want to use wine because of the alcohol you can sub in lemon juice or vinegar. Tarragon and dill are other herbs that are nice with different types of fish, something like salmon that has a stronger flavor for example. I have seen someone mention using blood orange juice in place of wine or lemon or vinegar.
You can reduce part of the poaching liquid [you can poach in milk and reduce it as well] to do a bit of a sauce, but I prefer not to.
[of course you can go the other way entirely and use clarified butter as a poaching liquid, a couple years back there was a slight fad for poaching stuff like lobster, shrimp, scallops and oysters in butter. Lobster poached in butter is great, but really intense.]