There are quite a few old threads on this topic:
How to invent plausible religions
Help building a religion
God and Pantheon Creation 101
Need some help with my gods
There are lots of textbooks on comparative religion out there too - two I personally rather like are The Phenomenon of Religion: A Thematic Approach. Moojan Momen (1999) and Reader in Comparative Religion. William A. Lessa and Evon Z Vogt (1956). Though often the most useful thing you can do with them is just look at the chapter and section titles and think about what (or whether) your religion has any important element of that.
Also, I think a lot of advice on this sort of thing is misaimed. Unless you are building a religion which is "true" and the story is going to be about cosmic issues the family tree of the gods and the details of their myths or deep philosophical discussions are not what you need. You want the stuff that outsiders can *see* - symbols, dress, food or other taboos, what the temples look like, key life stage rituals, what sort of things ordinary people might sacrifice, or do in the course of a day or a month or a year, when the festivals are, what happens on them, and five sentence or shorter summary of what they are about the locals will tell the foreigners. You need the little gods and saints that have street shrines or that people swear by, and the tradition of including an extra penny in any payment you make to be thrown into a well as an offering, not the "High Gods" nobody but scholar priests care about, or the once in a millennium Grand Royal Sacrifice. Recognizing those are different and showing it in your story is a great way of adding a touch of realism.