You're supposed to fill your deck with cards like in Hearthstone, but mechanics are a lot different.
In the upper left, you have the card's mana (souls) cost. Upper right, it's health. Works like in Hearthstone, meaning it does not regenerate end of turn. Lower left, attack. Self-explanatory. Lower right, armor. Whenever a creature takes damage, you subtract armor from damage and that's how much damage it takes. You get around this with poison.
Poison creatures apply poison stacks to creatures in addition to damage. A creature with one poison stack takes 1 damage on the beginning of your opponents turn, two poison stacks means two damage, you get the idea. There is no indication on how much poison stacks a creature has, so you have to keep that in mind yourself.
Here your creatures don't receive counterattack damage when they deal damage to other creatures, so if you're good enough, a 2-mana 2/2 with poison is just as good as a 4-mana 2/5 with poison. That's retarded and exploitable.
You're also not forced to have a 30-card deck to play matches. This can fuck up your deck if you don't know what you're doing. This is also retarded and potentially exploitable.
Awaken apparently activates your units. It's not like haste, meaning you can attack with a unit, use Awaken, then attack again.
High-cost units are a trap. Don't include too many unless you don't know what you're doing - you need low-cost minions to swing tempo in your favor so that you have mana to spend on, you guessed it, said high-cost cards.
There is no removal at the moment in this game save for Fissure, a 0-cost card that halves your HP (you start with 60-something, way too much in my opinion) and you generally want to bait it out since if you don't, you can get very, very rekt. It also can kill off your units for no reason the turn after you play it. That's retarded.
Beyond that, you're on your own. The game's half dead, as in I just waited 15 minutes in ranked matches matchmaking but got someone in this thread to play with me.
Also, the get 2 mana card is OP.