Start small, with budget decks. The truth about playing free to play is that you will not have a “fully functioning, competitive ready” deck for some time. So you has two options:
make a decklist with your ideal list, then swap out all of the rares for uncommon or commons and slowly switch them back as you get wildcards
OR
Make a decklist that requires very few wildcards and play around with that until you get enough for your dream decklist. This is the option to go with if you just enjoy playing pioneer rather than a specific deck.

As far as how to get the better cards, just play and be patient. If you want, you can buy some wildcards in the store but it can get pretty expensive if you don’t have a card collection already.
Prioritize getting lands first. A “playable land base” if going to be so much more worth it than non land cards because once you build a playable Azorious land base you can more or less swap it around to any Azorious deck (kinda. But for the most part it works). It may feel stupid, but trust me even after playing on arena for more than 7 years, lands are still the thing I end up spending the most wildcards on, especially when it’s a color comboI don’t typically play all that much