I primarily invest in ETFs instead of individual stocks, stock picking is possible but it’s very difficult and many professionals don’t even manage to outperform the general market over a larger timeframe.

For ETFs I would recommend you research one for a world index, of course the S&P 500 is a great option that many recommend as well but I personally don’t want to bet on just a single country.

For brokers, have a look if you can find any that are local to your country, banks also often offer investing services but will typically charge significantly higher fees than an online brokerage (you do get more safety in theory tho with a regulated bank). Alternatively have a look at your tax structure regarding investments and if it is manageable you could go for a broker like IBKR, I cannot personally recommend it as they don’t handle taxes in my country, which are a nightmare, but I have heard many great things about them.

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

A good tip for weapons is to check out the market tab in game, go to the weapons section and sort by only show with blueprint available. It will show a good number of weapons where you can get the blueprint for credits and you can check if you have the necessary resources to build it in advance.

Otherwise, you will unlock blueprints for weapons during missions and quests. Also look on YouTube how the relic system works, that will explain how you get prime warframes and weapons.

Well yes and no, in the end they will have to clear every star chart node to progress so that is also an option as the reason, even if there is no specific progress towards a frame or weapon.

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.