This is a (brief) primer, and doesn't leave much room for every facet.
There is a Veil separating the world we know from what we don't. Some say beyond it lies a spirit world/heaven/hell, others say different dimensions, still others say alien overlords. It's how we keep from seeing, and being able to cross into, whatever is beyond. A fair amount of Christians actually believe in The Veil, they just don't think of it much.
You can cross The Veil after death because you don't have a physical body. Some people get faint glimpses of whatever is next, which is why they tend to just know things.
There is an afterlife
Ghosts? Yup. A nirvana-like place? Uh-huh. Reincarnation? That, too.
Ghosts are those unable to fully cross or, at times, those who cross back-and-forth between different places. The ones who cross on occasion are generally messengers or those who feel a need to interact with people.
There is a final place, after all of your travelling is complete. It is a tailor-made paradise for each individual and doesn't adhere to one specific doctrine, Ghosts can't settle, so it is not possible for them to reach there easily. There is also a hell, of your own making, but it's more difficult to get into than your paradise.
And yes, animals exist as souls, as well, but I'm uncertain about them being reincarnated. Maybe it's attachment which keeps them eternal. Maybe it's the same for us.
Everyone goes through reincarnation. Your soul has more than one chance to live, but you're never "downgraded" into an animal if you were horrible in your last life. You are always human and your soul will often end up with (at least) some of the souls you currently know and/or love in subsequent lives. You must learn lessons, some more than once. I have no idea how many lives you live. And, it's possible that some spirits may choose to experience life repeatedly without paradise.
There are alternate dimensions
Every time you make a choice, the road splits and another one of you takes the path you refused in another dimension/plane/whatever. Each choice is another branch of your life. When you die, you get to see the tree you've made, the life you lived... or didn't. Too few branches, you played life too safely. Too many... possibly too reckless. But, you get to know.
Could you be an alternate self, stemming from a choice your "true self" made? Doesn't matter. Every alternate is a valid person, unto it's own pocket of existence.
God, and other "higher beings"
Is there a god? Yes. More than one? Maybe.
There are "higher beings" out there. The highest could be called gods. Most don't play by any rules we know about. Some will pick up names and wear them, answering to the call. Many of these experience their own type of reincarnation. Humans change our gods every so often, fitting them into how we view the world. The vengeful become forgiving. Males become females.
The image I keep of the god I pray to is both male and female, yet neither. It is compassionate.
There is no "right" religion.
Miracles, magic, and science
Miracles are just divine/otherworldly magic. Many things we thought were caused by gods have a scientific explanation, though it doesn't necessarily discount divinity. Prayers are a type of hope spell.
They're just different corners of the same sandbox. The problem is that most people think believing in one or two automatically makes whatever is left impossible. This is false. Some people say logic leaves no room for faith. Untrue. People who have faith often stop believing in anything else, which can be deadly. All of these things make (and partially explain) our world.
A side note: Science says the universe came about in The Big Bang, a tremendous explosion of brightness. In the bible it claims God said, "And let there be light". The bible was written before the theory...
One doesn't discount the other. (I'm using the bible because I'm most familiar with it.)
I could be wrong and... anything is possible
My beliefs have been slowly shaped by my experiences and research. I don't have all the answers. I could be wrong about everything, this is just what makes sense to me. Everyone has to choose what they feel is true.
I also believe anything is possible. Could it be there is nothing after this life? Sure. Could we be in a computerized reality? Maybe. But, until something comes along and changes my mind, here I am.