Start where the water temperature forgives you
Hojicha and bancha first. Both take near-boiling water, neither turns bitter easily, and hojicha in particular tastes nothing like the green tea most people have been put off by. Genmaicha belongs here too — the roasted rice masks the vegetal notes while you get used to the format. The point of this stage is to succeed at brewing before the tea starts demanding anything.
Then meet the two fixation styles side by side
Sencha and Longjing, brewed on consecutive days or together. This is the single most informative comparison in the category: steamed against pan-fired, marine against chestnut, and the difference has nothing to do with quality. Whichever you prefer tells you which branch of green tea to follow, and that is worth knowing before spending more.
Then explore within the branch you chose
If you went Japanese: kukicha and kabusecha, then fukamushicha for the deep-steamed style. If you went Chinese: Bi Luo Chun, Huangshan Maofeng and Anji Bai Cha, which run from fruity to orchid-floral to savoury-sweet. Korean green tea sits with the Chinese branch despite its geography, because it is pan-fired.
Then the demanding ones
Gyokuro and matcha, in that order. Gyokuro asks for water at 50 to 60 °C and several times the usual leaf, which is far enough from ordinary practice that it is easy to waste. Matcha asks for a bowl, a whisk and a grade good enough not to taste flatly bitter. Both are worth reaching and neither is a good first purchase.
Where it goes after that
Yellow tea is the natural next step — Huoshan Huangya beside a Chinese green shows what the yellowing step does in about two minutes. Beyond that, a lightly oxidised Baozhong or Tie Guan Yin is the shortest crossing into oolong, and it will feel like a continuation rather than a departure.