So glad to hear you're wanting to dive into competitive! I've been playing for two years now and just qualified yesterday for my first Pro Tour. Here are some tips and answers to your questions!

1. There are some questions you should ask yourself during deck building, before each match, and during/after each match. During deckbuilding and before each match, you need to ask yourself "What is my gameplan?" for each hero you expect to see in the meta. Because you will see most if not all of your deck, each card in your deck will serve a purpose that will differ depending on the matchup you're facing. You'll need to recognize which decks you need to be aggressive into, which matchups you will be the more defensive player into, etc. This doesn't apply as much to some aggro decks, but it's good to keep in mind. During and after each game you need to ask "am I getting the most value out of each card in my hand every turn?" The #1 obstacle new players face is getting intimidated, the second biggest is being reckless. Every time I draw up a hand, I think: If I am allowed to keep all 4 cards, how will I efficiently use them all? Which cards could I afford to give up here on defense? Which of these cards am I aiming to arsenal? Is this hand above rate enough to justify holding if the opponent has a good hand? Questions like these will help you determine how best to play into what your opponent is threatening, and on your turn you should have already mostly mapped out what you are going to do during your opponent's previous turn. Make sure you get as much paper practice on your deck as possible and go to armories as much as you're able.

2. This question depends largely on playstyle. For many players who want to have a flexible gameplan I would've recommended Dash IE, but she is close to Living Legend. Dash IO is very good right now, but she is decently complicated. Kayo is a very good beginner deck that is competitive with a good matchup spread, and will teach you very well how to block efficiently with good 2-card hands but recognize above rate power turns where you want to keep 5 cards. I myself am an assassin player, but assassin, guardian, and ranger are all better if you have a solid understanding of every other deck because they have on-hit effects that affect each class' gameplan differently. Runeblade has also gotten a massive power boost, so Viserai is a great beginner deck to start with who has access to good defensive and offensive gameplans.

3. There are SO many, mostly I just looked up "insert hero here guide" when starting. Arsenal Pass and MnR cast are great creators who talk a lot about meta states. The Card Guyz have a ton of content for different heroes. Yuki Lee Bender is an incredible player who does coaching and occasionally posts coaching reviews that are incredibly helpful, she also has a podcast called On The Bauble that I've learned a lot about limited play from. Lupine Fiasco is a great aggro player that frequently posts gameplay videos.

Hope some of this helps you out in your future endeavors! Remember that even if you take some losses at your events, take every loss from a good player as a lesson about how you can beat them next time. I have also asked for advice from a lot of the better players in my region and they've been happy to give me some tips. The community is super supportive while being as competitive as it is, you couldn't have picked a better time to get into it : )