Hey, welcome back!
I recently got back into it myself, and before doing anything I spent about a week researching the current metagame, watching videos, tournament results, etc. If you haven't been keeping up with the current game, I recommend you watch OmniPoke, google, ZapdosTCG and maybe YellowSwellow if you have the time. In terms of sites, there's, obviously, PokéBeach, and as for tournament results, you can check Limitless TCG. It's a particular good time right now because a big international tournament just took place in Brazil, so the results you get from there will be very much up to date.
Basically, the best decks right now are:
- Zoroark-GX variants (Zoroark/Control won the aforementioned tournament)
- Granbull: 1-Prize attacker that hits for 160 if you have no cards in your hand
- Buzzwole-GX/Diancie Prism Star (Landorus-EX 2.0, but I think you might know about this one)
- Ninetales-GX/Decidueye-GX/Swampert (aka BrokAn Deck, DeciTales): Ninetales' Ability gives the deck unlikely consistency. Between Ninetales and Decidueye, you can snipe up to 110 damage in one turn.
- Blacephalon-GX/Naganadel: The deck that everyone thought would be BDIF. You get Naganadels into play, accelerate Energy on them using their Ability, then you put the Energy in the Lost Zone using Blacephalon's attack. (Kinda like Magnezone Prime if you were around then).
Hope this helps!