Welcome. You can pick up a lot of basic info about tea here, to help you give your customers (and yourselves) a better experience!
Everything matters--the quality of the tea leaf, the quantity of leaf, the quality of the brewing water, the temperature of the brew, the timing of the brew.
Brewing if for customers, I imagine it's going to be harder to 'customize' your brews as easily as we can brewing for a few people whose preferences we can take into account (make mine 1/2 the strength of everyone else's, for example). Still, there are plenty of friendly teas that aren't so touchy about brewing conditions--e.g., that aren't lovely lovely lovely then suddenly BITTER! if you go a few seconds too long or use a touch more leaf than usual, and you can learn about them here. Have fun!