To get your hands on the Arena rewards, you first need to pay your admission fee. This will cost you either 150 gold or $1.99. New players are graciously offered their first entry for free from Blizzard. To make a comparison, a pack costs 100 gold in the shop and the Arena guarantees you at least one. So it will often be most "worthwhile" to start this mode and use your gold in the best way possible.
Once your pockets are empty, you have the choice from 3 heroes, offered completely at random. Pointless to hide it, certain heroes perform better in Arena than others. First of all there's the Mage who is very strong in this mode, mainly thanks to his heroic power that allows him to make an impact in the early game. Jaina has many powerful spells that are very effective in Arena. The Paladin is also one of the strong classes. He is very versatile and the balance between his spells and creatures will often make you happy. Generally speaking, the weakest classes are the ones that are very dependent on certain spells. The Shaman for example, will be very vulnerable without his key cards and deprived of his area spells. Nevertheless, all classes can be played in Arena and you can gain the ultimate reward with each of them.
Now's the time you will need a bit of luck and a lot of common sense! During the Draft of the Arena mode, you have the choice of three randomly offered cards. There are no rules in regard to this and sometimes you will have the chance to have some very good cards, or you will have to adopt lesser creatures. What makes the Arena such a popular mode is that all the players can start off on an equal footing because you don't need to have a card in your collection for it to be offered to you.
Unlike a regular deck, the same card isn't limited to two copies. If a good card is offered to you several times then you are free to choose it as many times as you like. Having good cards isn't everything and knowing how to build an Arena deck will be the key to winning successive victories. As such it is essential that you build a balanced deck and try to have a consistent mana curve.
In the Arena mode, the objective is to win as many victories as possible to get your hands on the best reward. The maximum number of wins you can reach is 12. This is your ultimate goal, allowing you to have the best key and open the most interesting chest. To reach your goal, you won't have to do it without a single mistake! You're allowed two losses in your path to the ultimate reward. The third loss brings an end to the Arena mode and it will be time to gather the fruits of your labour. In the worst case scenario you will still be given a pack and some gold or dust. The Arena is a calculated risk that most of the time should allow you to come out better off.