New players to Hearthstone always want to rush into Arena to get more cards, or Ranked matches in order to get to Legend as quickly as possible. There’s a reason why there are three modes of play in Hearthstone. You don’t go straight for Arena if you’re not very good at the game, and there’s no rush to play in Ranked matches either.
You can play against the AI, but in many cases you’re better off playing against other human players. You earn more experience points with each game, and you tend to learn how to defend against better strategies and cards. While the expert level AI players are solid, they’re still no match for a good human opponent. Start off taking all of the classes to level 10 by playing through practice to unlock them all, then switching over to Casual to get experience against human players. There are plenty of new and inexperienced players, so you won’t be losing all the time, no matter what your level of skill is.
Ranked matches require you to have a solid deck. If you only have a handful of rare cards, one or two epic cards, and no legendary cards, you won’t make it very far in Ranked play. It’s okay to hop into Ranked and see if you can make your way to level 19 or 20, but it’s around that point that you start to lose more often than not if you don’t have a solid deck.
It’s best to play Casual matches and complete the daily quests to build up your gold, then use that gold to play Arena matches. Once you have a deck with a good number of rare cards, quite a few epic cards, and at least a few legendary cards, then you can head back to Ranked and see how you fare.
When you’re played enough Casual matches to get all of the classes up to at least level 10, and you’ve learned enough about the game to win a good number of Casual (and possibly Ranked) matches, then you can head into Arena matches. You don’t need a great deck because you’re going to be drawing a random deck, but you do need good deck-building skills. These are skills you learn as you play each class and win (or loss) more and more in Casual play.
If you attempt to play Arena matches too soon, you’re going to lose early and it will be a waste of gold or money. You only have three losses before your Arena run comes to an end. To make sure the run was worth the price of entry, you need to win at least four matches before you lose three. Because of this you need to be able to build a good deck and win with it. Once you’re capable of doing this, Arena is the best way to earn new and better cards to be used in Ranked play.