To play right away: a Trial Deck
ETD01 (Red・Blue) and ETD02 (Green・Purple) are complete, legal decks straight out of the box, Soul Deck included. If your goal is to learn and play from day one, start here: two different Trial Decks already make a full table.
To build: the Booster Pack
EBP01 “Dawn of Palpagos” is where the depth lives: 161 cards to upgrade the Trial Decks or build new strategies. Buy packs once you know which colors and which game plan you enjoy.
To collect
High-rarity parallels concentrate the collectible value. Before paying aftermarket premiums, check which cards you are missing in Palpedia's Collection and compare options in the where-to-buy section.
Before you spend
Palpedia does not aggregate prices and sells nothing: tcg/deals.html lists the official products and tcg/events.html the demo stores where you can try the game for free before deciding.