First off, congrats on getting to rank 16! Not many players get there as fast as you did 👀
IMO, getting better at the game really helps you climb and enjoy the game more; having a stronger deck certainly helps but that's not something you can really speed up unless you spend some $$$.
Obviously you'll improve the more you play, but watching someone play is one of the best ways to learn. Many pro players, such as Firebat for example, tend to explain the decisions they make while playing. Honestly you can't ask for better tips than being inside the thoughts of a pro.
Another underrated way to improve is simply knowing your opponent. Sure, you can't choose who you go against but if you queue up against a Rogue for example, you can learn to expect fast paced decks, so mulligan in consequence. A great website to know more about what people are playing is HSReplay, there's a page with the most popular decks of each class and, on top of giving you new deck ideas, will show you what to expect while playing.