Most bitter cups of green tea are brewed with water that is too hot, not with tea that is too cheap (sample note).
Boil fresh water, then leave the kettle open: after two minutes it is around 90°C, right for oolong; after five, around 75°C, right for sencha. Black tea and pu-erh want water straight off the boil.
Every tea from our shelf comes with a brewing card. Start with its numbers, then change one thing at a time.