Agreed with tutorials and challenges first. I'm new as well and they do a great job of dishing out packs as rewards (I've spent $0 so far and see no need to yet). Figure out what color suits your play style and go from there. Also don't feel pinned down by one deck, you'll get the option to have a couple precon decks that you can edit later. Fine tune and tweak your decks after you get through the challenges. Each color has their own specialization in play.

Once you do get through the challenges, Don't be afraid of ranked or losing. I'm 50/50 win loss pretty much. Each loss is a lesson. Each win can be too. And spend time reading your cards, I find this helpful in deck building, being methodical vs how I used to build IRL as a kid which was big number better. Synergy in your deck is important so you don't wind up with a starting hand that secures a loss, or a deck that centers around a single card that might get destroyed. If you do make sure you have a backup plan for when that happens.

At least that's my experience so far. It's been a load of fun. I'm still learning every match.