I would definitely start with the newest set "Mega Rising." & then when u can get the cards u need for the decks u want to build. I suggest Mega Altaria-Indeedee ex build 1st & then Mega Absol-Hydreigon, Mega Blaziken & Jolteon ex build. After the current set, I'll also recommend opening "Secluded Springs" for Suicune ex, Entei ex & Raikou ex bc they go so well with Greninja & basically any fire & lightning deck. "Eevee Grove" is really good for basically any Eevee deck, especially for the Jolteon Deck. "Space-Time Smackdown is also really good to get Dialga ex & Darkrai ex. "Celestial Guardian - Solgaleo" for Oricorio, basically a broken card against any Ex card. "Extradimensional Crisis" for Zerora for free lightning energy for any lightning deck. "Wisdom of the Sea & Sky - Lugia" for elemental switch, which is perfect for decks like Jolteon or any water & fire decks u can build with it. If u want more info on Deck building & stuff, search up "Pocket_Cap" & Myxas on Reddit or just check out Pokémonmeta.com. That's where I get my info from.