Rank 15 puts you in the top 25% of Hearthstone players.  You’re Better Than You Think!
The pdf Tabby Cat linked to is pretty good. Much like in poker, most players never actually get beyond Level 1 thinking – I have this so I do this. Going to Level 2 and thinking about what your opponent has is problematical in Hearthstone because there are so many cards (and soon to be a whole lot more).  It takes a lot of study/time/experience but it isn’t actually all that difficult, just time consuming. That’s a good reason to study the popular decks; be familiar with what is most likely to happen rather than spending a lot of time thinking about things that are very unlikely.
I don’t play much anymore – usually just do the daily, maybe play an hour or two once in a while – and end the seasons at Rank 5 or better … just haven’t put in the grind time to hit Legend.
There are always a bunch of people streaming Hearthstone on twitch.tv; filter out the cursing, drunken young males with loud rap beats drowning out their chat and most of the tittering young females. It’s a very slow process learning by watching but I did find it helpful in the beginning to watch people like Trump, Kolento, and Reynad (when he’s sober) and listen to their thinking.
I’d say that studying and playing the decks of other people is a fine way to learn how to build your own. You didn’t do it alone in poker, you read the books, right? Keep in mind that those top level deck builders have been playing the game for over a year – use their experience.
The deck listings on Hearthpwn are a good starting place. You can filter out the cards you don’t have to find decks you can make. A lot of the deck listings have good discussion on why certain cards are included or excluded, and on what cards may be acceptable substitutes for ones you don’t have.
For climbing the ladder, aggro Hunter is the fastest and easiest to play. General Hint: concentrate on clearing his board in the early game, then hammer his face once you get him down to 15 health.
Warlock Zoo is a bit more difficult to play but is probably the most common deck for ladder climbing. It’s not really aggro, it’s more about making favorable trades than about bashing his face. Zoo decks can be very cheap to make, consisting mainly of basic cards.
Hunter and Zoo decks can climb to Legend. You don’t need a big collection of Legendary cards.
To reach top levels will take a LOT of experience mainly because there are so many different cards to consider. It will be very time consuming. And they keep changing the rules.
Two very important things to think about while you play: If you are going to do something that draws a card, draw first (if you can). Always count to see if you have Lethal.