A bagged Tetley Pure Green or Twinings Pure Green at 80°C for 90 seconds. Both are clean Chinese style greens, low astringency, no bitterness if you respect the temperature. Once those become normal you can step up to Sencha, then a single origin Japanese green, then matcha.

Roughly 30-40mg per 200ml cup against 40-70mg for black. The difference is small enough that "I switched to green tea to avoid caffeine" is mostly placebo unless you were drinking a lot of strong black before.

80°C, not boiling. Boil the kettle, then either let it sit for 2 minutes or pour into a cold cup before brewing, both drop you to the right temperature. Above 85°C the catechins (the bitter compounds) over extract and the cup turns harsh.

Powdered green tea is "matcha" only if it is stone ground Japanese tencha leaf. The cheaper "matcha" you find in supermarkets is often powdered Sencha or a Chinese equivalent, it tastes flatter and lacks the umami sweetness real matcha gives. Pukka Supreme Matcha and the Teapigs matcha sachets we stock are both real matcha.

There is some evidence the catechins (notably EGCG) modestly increase metabolic rate and fat oxidation. The effect size is small, a few extra calories burned per cup. Drinking green tea instead of a sugary drink is a much bigger win than drinking it on top of one.