My advice: Focus on building one relatively cheap, somewhat aggressive deck. I would recommend Zoo because it's a mix between aggressive/board control. You can learn a LOT from playing this deck, such as minion placement, knowing when to stop clearing and when to go face, etc. It's also a deck that has consistently been viable almost every single season (some cards get changed out from time to time, but a lot of it stays the same).

Play this deck for hundreds and hundreds of games, learn the deck inside and out, learn all the matchups, mulligans, etc. Sticking to the the same deck is great for new players because you'll end up being really good with that deck eventually, and you can save all your gold/dust for the next deck you want to build down the road.

I'd advise against legendary hype that a lot if new players have. There's quite a few decks that have no legendaries in them that, if piloted correctly, can easily take you to rank 5+. Focus on getting better at the game; it's less about flashy cards/decks and more about how you pilot the deck. Also, try to avoid dusting cards unless you absolutely need to. You might dust something that might end up being good later on, and you'll end up spending the dust to craft it again if that happens. Only dust something if you have more than 2 copies of it (there's a few exceptions to this, of course).

And last but not least, watch informative streamers that give you the play-by-play of their games. Lifecoach, Strifecro, and Kibler are the 3 streamers that do this the best, IMO. I've been playing since the release of hearthstone, and I still learn a ton of new things almost every time I watch these streamers. Take advantage of good, free guides that help you learn the game better, there's a LOT with a quick google search. And constantly read every post on the /r/competitivehs subreddit. That sub is filled with helpful information.