One of the first and most important things that you'll need to attend to in order to play a game is to to construct a Deck. A Deck is a comprised of picked cards from your library of available cards - which makes up the pool of cards you basically draw from during a match. If you are not familiar with these mechanics similar to almost any card collecting game, there is the option to immediately dive into the battles as each Hero has access to a pre-tailored deck which should suffice to get you familiarized with the basics of the game.
On the image above, you can see the game's deck-building interface. It is pretty much straightforward and very intuitive. The values 0-9 and a bar graph above them (on top of your current total "cards-on-deck" value) depicts the balance of your unit cards with respect to their summoning cost (Gold Cost) - as higher costing cards tends to take time to summon, it is more likely that you focus a larger sum of lower costing ones than the higher costing ones.
From this menu, you can also access your crafted cards - which will be discussed on another section in this guide.
On the other hand, if you wish to customize your deck, it will be subject to the following rules:
1. The deck must contain 30 cards, no more, no less.
2. Certain cards can only be used by certain Heroes.
3. Decks cannot contain more than 2 copies of any cards of Mythic rarity or lower.
4. Decks cannot contain more than 1 copy of any cards of Fabled rarity.
Other than that, there's probably no limit to how you could customize your own deck and make it more effective than your opponent's. To boot, a custom tailored deck can also be saved like a profile from which you could always recall when needed.