There's an x mark on your quests. That lets you reroll your quest into something different. You can maximize your gold by always rerolling any 40g quests.
Try to focus on classic packs first. Expansions usually roll over into the Wild format every 2 years but the classic set is (mostly) evergreen. Once you get a comfortable base of Commons and rares in the classic set then start picking up cards from the most recent expansions. Like Whispers of the Old Gods will rotate out of Standard in this coming April or so, but Un'Goro and Frozen Throne will stick around until early 2019 so it is safer to collect from these expansions.
It will take a long while for you to get "caught up" to people who have been playing a lot longer and have more cards so just keep that in mind. Ranked ladder will likely be a pretty lopsided affair for you because ladder placement is more about how often you play rather than how good you are. So even in the low ranks you'll be up against people with much fuller collections. So casual will give you the better experience because in casual you will be matched up based on winrate.
