I'd suggest playing on MTG Arena because you can play with a lot of people at your own pace to relearn the game and the game enforces the rules so you can't get them wrong (but if you don't get why something happened, you'd have to go ask people).
I would not start by buying boosters if you're starting from nothing because you get such random cards that it would be difficult to make a deck that works for anything other than Sealed or super casual. I would probably pick up preconstructed decks, there are a few different product lines. The Challenger Deck product line consists of versions of competitive decks for Standard (which is a two-year rotating format); the 2020 line is
here.
What are specific questions you have?