Depends on the exact country, as a local broker will (usually) handle taxes automatically for you. Of course you can do them yourself but in some European countries this could be time consuming and complex.

The EPA states that each American uses an average of 82 gallons or 310.4 litres a day (study from 2015). Source: https://www.epa.gov/watersense/statistics-and-facts

Your main priority should be to progress through the star chart and clear all nodes, while doing that you will continuously unlock new sections of the main story, complete those as well.

This will slowly unlock all the (almost) endless content, game modes, frames and more that warframe has to offer.

Try out new weapons and frames along the way and always try to level something on the side so you keep increasing your mastery level.