It's the cleric equivalent of fireball. An almost overly strong spell that is basically a must-take and must-use as soon as it becomes available. It's almost always the best thing a cleric can concentrate on.

For sure. Just bear with me, this is gonna get a little long-winded. Like, too-long-to-fit-into-one-post-on-Kbin long ;p

A lot of it depends on party make-up and playstyle. I'm gonna assume your party is rocking with the usual set-up: two melees, a ranged character, and a healer/spellcaster (you).

First thing to remember is that, as cleric, you kinda sorta have more responsibility than anyone else in your party. For instance, all the barbarian has to do is rush the bandit and swing his axe. As cleric, you're gonna be expected to attack, heal, buff and debuff. To be a great cleric, you’re going to have to pay attention every round—see who needs healing or a spell to cure frightened/confused/what have you. Some people don’t like this responsibility but I absolutely love it.

Your position on the battlefield depends on playstyle and party. If your party is configured anything like the one I mentioned, then I think your best bet is to be between your melee and ranged allies. Basically, you want to be close enough so that your spirit guardians can really mess with your enemies but not so close they can fuck with your concentration.

SPELLS TO CONSIDER:

Other than the ones mentioned before, here's a few more…

Healing word: use it sparingly, mostly to bring your allies back to life. There's no point in wasting the slot if your ally just got nicked for 5hp. Save it for when they lose consciousness and are on the brink of making death saves.

Aid: Great for a majority of the game. Personally, I like using a third level slot to give my allies +10hp. Because the health from Aid ISN'T considered temporary, the extra temp. hp from Twilight Sanctuary stacks with the hp from Aid. So, with aid and TS, at low levels you can almost double your party's HP.

Spells gained through twilight domain: You gain access to some of the best spells IMO. Faerie Fire, Moonbeam, See Invisibility, Leomunds Tiny Hut, and Greater Invisibility are all great spells. Pair moonbeam with an ally who has booming blade and bosses (aka your DM) won't know where to go.
Put greater invisibility on your rogue (or druid) out of combat to steal anything you want.
Use LTH whenever your party needs a rest, and even as choke points to mess with your enemies (they can't go through the hut, neither can magic).

...

"Passive" skills:

Eyes of Night: So much for having to worry about dark vision. Having this pretty much allows you, and everyone in your party, to pick a race that doesn't have access to dark vision. The game hasn't even started and you're already helping out your buddies.

Vigilant Blessing: Advantage on every initiative roll you–or anyone of your choosing–makes. This is huge. Just make sure you use it.

Twilight Sanctuary: This is what makes you god-like. In my opinion, it's the strongest skill in the game. Time it right and you'll mow through bosses. Pair it with spirit guardians and good luck getting the smell of dookie out of their pants.

Steps of Night: Useful at night and in dungeons. Aesthetically badass (imagine being a bandit and seeing a heavily armoured dude with wings slowly floating towards you with two kickass spheres ((TS and SG)) surrounding their party). Also great if you need to flee a battle.

You can't cast two leveled spells. But you can cast a cantrip and a leveled spell I believe

https://rpg.stackexchange.com/questions/49532/casting-multiple-spells-in-a-single-round#49533

That's how it works yes, but any rule can be vetoed by their DM if they prefer to play like that

Yeah it's a great spell. It scales well with upcasting. It also doesn't require concentration which is super helpful, especially for a Cleric