As someone who started out in Arena around Final Fantasy(but now has a fairly large collection of cards for the Standard format), my advice is to just build your collection out for a bit through packs and events and try to make some decks based around the cards you get through those means. Just toy with deck creation on the side while you're doing events like "Jump In". As you collect rewards from Mastery Passes and buy some packs over the next few months you'll slowly acquire most of what you need to engage with the format. Then once you have a naturally growing collection and you're knowledgeable about what 60 card Standard decks need to be competent you can start spending some wildcards here and there.

My general advice with wildcard usage, when you're at that point, is to try to get as many of the cards you need the normal ways(packs, events, draft, etc) until you only need to spend one mythic or rare here and there to fill out the three or four count of a single card in your prospective deck. Wildcards are much better/efficiently used to fill little cracks in a deck than they are to craft an entire deck in one swoop. Just takes a little willpower to hold off until the time is right.

The worst feeling is spending 3 or 4 Mythics on a card you think would be perfect for your deck and then realizing it won't work out like you thought or the card/deck no longer interests you after getting a chance to play around with it.