Where do i go to find out about spectral guardian AC, Attacks, and the like?

It doesn't. It's more like a stationary damage spell in a form of a guardian. More of a crowd control, since it stays in one position the whole time. 

Just checking ... hostile targets get hit each round (one time per round) so long as they move to a space within 10’. So you can be in the zone and take no damage so long as you don’t move ... or move to a space more than 10’ from the Guardian?

Also because Guardian is described as Large ... does it “occupy” a 10’x10’ space?

It does occupy the space, according to the spell, and yes, it only attacks a creature that moves into a space within 10 feet of it. Note that it doesn't have to willingly move there it can be dragged, pushed, or otherwise forced into range, and the guardian will still attack. Furthermore, it attacks once per turn, not per round, so if you drag someone into range, it attacks. Then, on the next person's turn, they move anywhere in its range, it will attack again. 

Good call re turn ... and yikes!  But I wonder if your point about being dragged / pushed / forced into the zone applies? In the spirit of the AoO rules is there a distinction between a creature moving vs being moved?

”You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction. For example, you don't provoke an opportunity attack if an explosion hurls you out of a foe's reach or if gravity causes you to fall past an enemy.”

True, but it's exactly because of that distinction that I believe the guardian attacks things that are forced into moving. It isn't taking an attack of opportunity (since AoOs are usually given when a something LEAVES your reach, and the guardian is attacking when something moves INSIDE of its reach) so much as it is holding attacks for when an enemy gets close enough, and those CAN be triggered by forced movement. Since the spell doesn't  specifically say it has to move willingly, like booming blade for example, I take that to mean any kind of movement. Just the ruling I go with at my table though. That's the joy of D&D. Your table can figure out what you think makes the most sense, and go with that. If you're the DM, then its up to you! And if you're a player, talk with your DM about how it works before grabbing it. 

Good call. I started a campaign with a new DM who said “I rent to allow stuff that sounds like it’ll be awesome” ... joyful indeed.

my cleric wants to put up multiple guardians everytime the party takes a long rest ... seems kinda overpowered, but the spell description doesn't say that he can only have one ...

ATM I'm considering to only allow one guardian OR have the guardian disappear once the cleric falls asleep... what do you guys think?

That honestly doesn’t seem that bad. The guardians are totally stationary, they don’t alert anyone sleeping. They’re also large, occupy space, and ostensibly look very out of place, which doesn’t help if you’re trying to sleep and not be found. If something were to happen at night, the cleric wouldn’t have these spell slots available.

There’s lots of creative ways to punish this tactic. But also around this level, 5e seems designed to set up the party for safe camping anyway.

thx for your reply. this seems reasonable.

If the guardian occupies a space, a hostile creature can't go through that space, right? 

Half of the things that get this spell don't necessarily even have a deity. For the warlock, the patron could make sense, but the paladins? Whatever it was you swore your oath to?

Quote from ClericalKenku >>

Half of the things that get this spell don't necessarily even have a deity. For the warlock, the patron could make sense, but the paladins? Whatever it was you swore your oath to?

 All of the classes that get this use divine magic, I'm not sure what you're talking about.

Stando powah

What happens if you cast this at a higher level? Does it go away after doing 60 + 20 for every spell slot higher used?

What if multiple creatures spawn or enter within reach at the same time? Do only three random (or even chosen) creatures get hurt, or do I get to damage all of them?

He's talking about the line "...emblazoned with the symbol of your deity". Divine magic also doesn't always mean gods. Celestial warlock isn't always a god like entity and Paladins don't even need a 3rd party,  probably what's he's trying to point out.

Can I use this spell to cast to guard my cart if have to leave it to investigate a cave or something and if someone triggers the attack will it let my character know like a car alarm even might be out of sight range?

Could this be a mount?

probably not. It doesn't say it can move, and it's also spectral.