Thanks, glad it's been helpful. I would also like to recommend Rubicon by Tom Holland, though not for any grand reason. I first read it fifteen or so years ago, and I remember it filling me with a sense of ambition. I don't quite know why, as it is just a (very well written) narrative history. But I enjoyed it thoroughly, and decided to reread it again last month. Those same productive feelings have returned, and I hope they might benefit others too.
>where should I start?
The computer is the botnet, lad. That's the hardest pill to swallow. I could advise you to spend thousands of hours researching how to administer OpenBSD, set up your own network infrastructure, use PGP, etc, all for the purpose of navigating an increasingly shrinking portion of the internet. But all of that is the wrong answer to the right question. Life passes you by while you stay locked in your bedroom trying to find the 'right' way to interact with big corporate websites and their consumers.
Download and install Fedora, it's solid and no-hassle at all (there are other perfectly acceptable distros as well). Use whatever browser you want (other than Chrome). Use the computer to get things done, and then go spend the rest of your day living a fulfilling life. Exercise, read Ted, learn a wide set of employable and personal skills, manage your finances well so you can retire early on passive income, help your community, find meaning in life. Allocate a brief period of online leisure in the evening, and then go to bed.
Systemd has no bearing in the real world. You won't spend your final hours in a rest home wishing you had found the right tiling window manager or playing more video games. These are all distractions; an illusion of choice that keeps you miserable and mired in trivialities while unable to grasp the bigger picture. Show me anyone that spends a significant amount of time online, and I will show you a basket-case of depression wallowing in meaninglessness.
>stop being underweight
If you mean physically, that's pretty easy if you're willing to work at it. It will improve your self confidence and you'll feel immensely better about yourself. I would just need to know where we're starting; rough age, how underweight are we talking, what is your diet like, are you a cripple, etc?