I have advice, too, which is: Don't. Even if you do, for whatever reason, start with the world and not with the story. You're starting with an assumption, and one which will constrain you. Your society does not need to have a creation myth. And even if it does, it doesn't have to be important in the practice of the religion.
The first question I would recommend is to start with the society in hand and ask what kind of religious rituals are practiced. You could also go by "What do the religions teach?" but that might lead you to make assumptions too quickly. (Easy to fall into assuming that the religion is like yours.) I still have unfond memories of someone who would post that all religions taught basically the same things, and gave a laundry list of precepts as if religions were ethical societies. If you look at the core of Buddhism, you find the Four Noble Truths, and ethical conduct is one eighth of one of them -- the Eightfold Way includes Right Conduct. And that's a religion that treats ethics as a high priority. Take as a converse the gods of Homeric Greece, where a hero had to take an oath by the gods that he was telling the truth, because the gods would overlook lying but objected to taking their names in vain. (Not, mind you, classical Greece, where an anecdote was told of a priestess who urged her son to avoid politics because if you told the truth, men would hate you, and if you told lies, the gods would hate you.)
Myths are also bad because the connection between myth and religion may be scanty. Notice that Plato takes it for granted that the myths can be jettisoned without difficulty to prevent their teaching the young bad things.
So start with their religious practices. Is there a specialized priest? Or are rituals performed throughout? If, for instance, your culture practices ancestor worship, the closest you would come might be to have the head of the family performing the rites. Not necessarily -- there may be priests appointed by the king or somebody to perform rites for "hungry ghosts" that have no living descendents, to propitiate them. Bitter accusations may be flung about your neighbor's failure to appease his own ghosts.
At which point I bring up that there are two kinds of religions. One is the kind that is so thoroughly inter-meshed with its society that its practitioners are not aware of it as a religion. Shinto and Hinduism are two of these. It is worth noting that neither of those had a name until Buddhism came along for people to become aware of the contrast. These tend to be highly syncretistic, dragging in all sorts of gods and other elements, the more the merrier. If you go for this, religion is not something your characters will be aware of as a concept; they are not in the habit of abstracting it out from life. Furthermore, it will not have a history from the eyes of the believers, except in unimportant details. The central core of it is that this is what has always been done. (Judicious examination of religious texts finds this belief is often -- less than justified.)
One thing you will not find with them is that a character picks a god and worships him, rather like joining a sect. It is true that you would not practice all the rites, but that's because not all the gods are relevant to your situation. A wife would propitiate the love goddess to keep her husband from wandering, the childbirth goddess to ensure she and the babies survived, the fertility goddess to ensure she became pregnant, the child god to protect the born children -- but not the ocean god because she does not go to see. Or the gods might be the gods of location, and you worshiped where you went, and ignored those you didn't. Of course, you wanted to worship any god who might get into a snit about your failure, and certainly any god who had gotten into a snit, as judged by your misfortunes.. The Athenian altar "to the Unknown God" stemmed from an attempt to divine which gods were angry and make an offering. Similarly, the Athenians treated insanity with rites to various gods known to cause insanity; when recovery did not occur, it was obviously time to go on to the next because it wasn't this god who caused the trouble and so could purge the ritual pollution that caused it.
And then there are the other kind, that are forming in contrast with another religion, whether the first type or another of this one. Obviously, since this will require reject of some views -- to make a difference -- it has less tendency to slop about and drag in all sorts of things. Buddhism, Christianity, Judaism are three such. Monotheism tends to be this kind. If for whatever reason, you want philosophy in your religion, to, say, have it center on the Good, the True, and the Beautiful, it probably wants to be this one. It does not mean that it has to keep a rival about; such religions can triumph. However, it will have a history. It will be aware of a founding, and probably of the times of darkness before the light of truth became known.
Meanwhile, there is the real possibility of divine beings wandering about having a hand in things.
I recommend their not having too much control of it. For one thing, bringing the gods on stage and having them shove things about tends to dwindle them. They are then openly and manifestly characters like the others. A god can appear greater than the surrounding most easily by hinting at things rather than being direct.
As for the gods that resemble three-years-old who are badly brought up, let's skip them. For one thing, if they are the Powers That Be of your world, such infantile gods argue for an infantile world. For another, even the myths that show the gods behaving badly don't show them like that, so it makes it obvious that the writer is driving it. For a third, if the human characters are to be better than the gods, shouldn't you at least make it a challenge for them?
I note that the polytheistic gods are not in direct competition with monotheism, because monotheism teaches a God who is a different order of being that His creations. A polytheistic god is, at most, an ens summum or highest being, not an ipsum esse, the subsistent act of being itself. An atheist superhero in the DC universe quite rightly dismissed the existence of the pagan gods as real beings as irrelevant -- after all, they were obviously the same order of being as Superman, not transcending him.
Part of bittercon.