Hi b_squared,
I started back 2 months ago and got some great advice here.
Firstly are you wanting to collect or play? If you're playing I'd advise you do what I was told to and buy 2 mach strike theme decks. You can take the 4 Cynthia, 4 garchomp,gabite and gible and 4 riolu, 2 lucario to make a fun practice deck.
Then I'd strongly advise researching on YouTube, pokebeach and generally on the internet for the different decks out there to see what you'd like to build. There are staples you'll want such as Guzma but again a bit of reading will make that easier to see.
Finally I'd buy singles rather than packs or boxes, I don't know where you're from but in Europe there is a website called card market
https://www.cardmarket.com/en/Pokemon
I'd also recommend using that to buy singles if you are collecting.
Send me a DM if you want to discuss further pal, it's a massive world with so much to learn but the community is great.
Good luck pal.