Alright, I'm gonna go through all the C'thun support cards and give my opinion on whether they're worth running or not. Only one or two of these are possibly competitive playable, but since you're a new guy, it's much more important to build and play the best deck you can with what you have instead.
All C'Thun cards can be found here
NEUTRAL CARDS
Beckoner of Evil: a 2 mana 2/3 is fine, but you can easily use other cards. It's functionally the same as [River Crocolisk], and the slot would be better filled by [Flame Juggler].
Twilight Elder: again, a 3 mana 3/4 is perfectly fine, a little above the average card at that cost. The thing is, most 3 drops have a useful effect tacked on to them, usually only costing 1 health OR 1 attack. See [Squirming Tentacle] and [Earthen Ring Farsser] for examples, but in the current meta, a 3 drop with 4 attack is more important, due to cards like [Tomb Pillager] and [Flamewaker].
C'Thun's Chosen: This is where we start getting good with neutral cards. A 4/2 for 4 is a bit below curve, but divine shield is a very powerful effect, and effectively makes this card at least a 4/3, since the shield will eat at least 1 damage 99% of the time. The closest comparable card is [Silvermoon Guardian], but as discussed in Twilight Elder's explanation, 4 attack is a good bit more useful than 3, though 2 attack means it risks being removed by 1-mana creatures.
Skeram Cultist: The last description was a bit long so I'll keep this shorter. Ahead of curve for a neutral, but most 6-drops will have more powerful effects that put them above a vanilla body. Compare [Venture Co. Mercenary] and [Drakonid Crusher].
Disciple of C'Thun: a 3 mana 2/1 is kind of horrible, let's be honest. there are 1 drops that are more powerful than that body, but the 2 damage it deals is surprisingly relevant, and can finish off problem creatures while ignoring those pesky taunt creatures. Compare [Argent Horserider] and [Ironforge Rifleman].
Twilight Geomancer: a 2 mana 1/4 with taunt is just bad, let's be frank. 1 attack is particularly bad because it can't kill most anything that sees play that costs more than 1 mana. Compare [Shieldbearer] and [Sunfury Protector].
Doomcaller: This guy is bad if you aren't running C'thun. Like, actually really bad. One thing to note is that most every big minion you'll actually play against will have around 6 health, at least currently. Compare [Bog Creeper] and [Eldritch Horror].
Crazed Worshipper: [Fen Creeper]. It's a solid taunt card and a nice option for new players, but there's literally no reason to play CW over Fen, since you can't pretend you have C'Thun (the animation pumping him doesn't trigger if the almighty 10-drop isn't anywhere in your deck.)
Twin Emperor Vek'lor: Stop.
CLASS CARDS
Dark Arakkoa: The important thing to note here is that it has 7 health, which most decks cannot easily deal with using one card. Combine that with 5 attack and taunt, and it'll almost always take down a couple creatures, and gives you a bit of security when you drop it down, ahead or behind. Compare [Sunwalker] and [Ancient of War].
Klaxxi Amber-Weaver: No reason to run this over [Chillwind Yeti], which is actually one of the best minions the game gives you before you even open your first booster. That said, no need for 4 of them in a deck.
Hooded Acolyte: Considering a 3/5 for 4 is considered playable in current constructed given a useful effect, a 3/6 is totally acceptable. There are some cards that are easy to obtain and have a bit more impact on the board. Compare [Holy Champion] and [Hungry Dragon].
Twilight Darkmender: 5 mana 6/5 is right on curve, but as usual, we have a vanilla body, so there are going to be options with effects that are usually better. Compare [Kvaldir Raider] and [Pit Fighter].
Blade of C'Thun: an [Assassinate] strapped onto an [Ogre Magi]. Seems fine for it's cost, but the cost itself is too prohibitive. By the time you reach 9 mana, the game will be at least half over, and just destroying a creature isn't going to be enough to turn the game around. No reasonable comparisons in a nearby cost, but [Icehowl] is worth looking at for something a bit more playable (please do not craft this if you're new, thanks :) )
Usher of Souls: [Pit Fighter]
Ancient Shieldbearer: [War Golem]
Cult Sorcerer: Compare to [Kobold Geomancer] and [Dalaran Mage]. This card is straight up better than both of those options, so if you're looking for spell damage, this is an amazing choice. Whether or not your deck needs that effect is at your discretion.