The changes that make the most difference
Use more leaf and steep for less time. Get the water temperature right for the category rather than defaulting to boiling. Give the leaf room to open. Take it out when the time is up. Buy smaller quantities more often so what you are drinking is fresh. Those five cost nothing beyond attention, and between them they account for most of the gap between a disappointing cup and a good one.
Then: better leaf, bought in smaller amounts
There is a real quality floor that technique cannot lift. Once brewing is right, the leaf itself becomes the limit — and the most efficient way to improve is to buy less, more often, from someone who states origin, harvest year and processing. A 50 g bag of something specific will teach you more than 250 g of something generic, and you will drink it while it is still fresh.
Then: brew the same tea two ways
The fastest way to develop judgement is comparison. Take one tea and make it twice, changing exactly one thing — five degrees, thirty seconds, a gram. Or brew the same tea western-style and gongfu-style on consecutive days. You learn more about what the variables do from two cups side by side than from any amount of reading, this guide included.
Last, and least: equipment
A gaiwan, a scale and a variable-temperature kettle are all genuinely useful and none is necessary. Buy them when a specific limitation makes itself felt — when you are guessing at temperature and it matters, when you want to run a multi-infusion session, when your eye for leaf quantity keeps being wrong. Buying equipment first is the most common and least effective way to try to improve tea.