Don't forget that most of religions are escapist fantasies at its base. Like science nowadays, it was used to understand the world around people. What we can now explain with lightning formation in the clouds, they understood as god's (or gods') wrath.

Gods - as opposed to mere spirits of things - exist in religion because human beings are flock animals, however unwilling we might be to admit both of those qualities. We like to be told what to do, to relinquish ourselves of responsibility, and god/gods is a perfect excuse for that - right alongside fate, destiny and other determenistic/oppressive concepts. Gods may start as spirits of things - spirit of the lake (or, later, of all lakes and, even later, of all visible outside water), for example - but grow in power with time if left unchecked (and most are).

Some people also enjoy abusing the power a religion often grants. Thus, come priests (who, let's face it, aren't always the benevolent wisemen we see in fiction); thus, come bishops; thus, comes Pope and other such figures - the "representatives of God on Earth", those who can "hear God's will" and so forth. If a lightning strikes a tree and everyone's terrified, it's the easiest of opportunities to seize their attention and wield their minds: come forth and declare "Lo! The All-Capable One spoke to me! He declared us wrong-doers! We must now repent! Chastize, brothers and sisters!". Case solved; flock organized.

Lastly, keep in mind how religion often circumvents attempts at logical explanation. I'm now going to overly simplify human nature for the sake of an example. Those affected deeply by the religious outlook (God made things, God takes things, I am a servant of His, I shall earn for good and be punished for bad etc.) will hardly if ever admit that there's often a more logical, reasonable explanation to whatever happened. A child murdered? Two options are most prominent in such situations: either the child fucking deserved it (sinner of high order and, coincidentally, people never liked the kid) or the murderer is a fucking monster. At no point will there be a place for a proper explanation: that the child was a manipulative little bastard at their young age and that they've been blackmailing the future-murderer into submitting to their petty wishes... and so on. It's not a solely religious thing for people to coward behind a brickwall of a simple non-explanation, but religions often give more solid, more opaque walls to hide behind in their fundamentalist conviction.

That's for psychology behind religion. Other aspects - number of gods, their importance, their genders and so on - are culture-dependant: develop the culture, and alongside it, develop religion to match.

For proper language development, linguistic knowledge would be of great help. Thankfully, you can now study linguistics online, at your own pace, no homework required, from various sources. Beside that, take a look at how John Tolkien, an English-linguist himself, developed his languages. One thing that stood out, for me, was how he gave a verb in an Elven language a recognizable, but different past tense form. An example (making one up) would be "alar" (present tense) - "kalja" (past tense) - "alanur" (future tense). The justification for that was that, just like Earthen languages, Middleearth's (or wherever Elves of his hail from) developed over time, and uncommon, irregular forms gathered up for one reason or another.