Mage is probably the best class for the fundamentals of board control, trading, area of effect, and everything, and it's also nice because the majority of its good cards are basic so you don't need to craft them. But you're going to open a bunch of packs in the first few days, and from there you can see what class you have the most cards for and may want to try. Of course, if you don't like that class, then you can play something else. You probably don't want to disenchant anything until you have enough to make a whole deck that you want, since that plan could change while you're in the middle of crafting it, and then you would have wasted dust or cards.
What you want to do as a beginner after the tutorial:
- Unlock all of the classes in practice mode
- Beat all of the expert AI
- Get all classes to level 10
- Do the first quests that it gives you that are like "play a game in ranked" or "win 3 games" or whatever
- Always play arena instead of buying packs. Even though it costs 50 gold more, it'll make you better and it's easy to earn your money back with arena. Depending on how well you do, you get a certain amount of rewards after your arena. You'll always get at least a pack and a bit of gold. If you get 3 wins, you'll get at least the 50 gold back, and if you get 7 wins, you'll get all 150.
- Play through Naxxramas and unlock all of those cards. If you're not aggressively Free to Play like me, then this is the most efficient place to spend money in game, since you get the best exchange rate with respect to gold. If you do want to get it with gold only, then I would probably wait until you can consistently get at least 3 wins in arena, because the cards are useless if you can't play them well, and arena will get you other (not guaranteed) cards
And after a bit of time you'll get into a cycle of play quests and stuff in ranked mode so that you can earn gold to play arena so that you can get better cards to win more in ranked so that you can get gold more quickly for arena and so on.
Some fairly cheap decks to think about making at some point, since you'll be interested in playing more than one class:
Zoo Warlock: This is just based around flooding the board with cheap, efficient minions to control the board and overwhelm your opponent quickly. You'll get good at how to trade and position your minions. If you've played MTG you'll be familiar with the idea. You can look up decklists, they all vary a bit, but they have a core group of minions. The preferred list will also probably change by the time you make this. But the deck itself is always good and relevant.
Midrange Hunter: This is the cancer sweeping the ladder, but it's also probably the best deck at the moment, and it's very cheap. Decklists also change from person to person, and Blizzard may nerf the class a bit, so I'm not gonna give you one specifically to make, especially since I don't like to play this class/deck. But you can look it up.
Additional resources:
http://www.reddit.com/r/hearthstone/comments/1ua5c9/hearthstone_for_beginners_guide_and_resource/
This basically says what I just told you, but it also has other helpful links.
http://www.twitch.tv/trumpsc
Trump is probably the best streamer to watch when you're trying to get better because he explains every turn and is also a fun, relaxed guy in general.
You can also add me and ask me about anything if we're both online at the same time, my battletag is LonelyLink#1633.