And yet another user who could vastly benefit from the vast amount of knowledge that would be compiled and discuss
if the forum were to house a Cybersecurity & Privacy board.
Achieving anonymity and privacy with peace of mind that you're doing it right requires a lot of research. It's a personal experience and it varies from person to person based on their threat model (who are you protecting yourself from?) and their knowledge-base (what practices you know of, what you can comprehend and what you are capable of employing). Discussion helps to learn more about both of these topics, which is why I am hoping the aforementioned board is added.
To answer your post, two basic rules:
1. Never share your personal information online unless you absolutely need to. If you do absolutely need to, do so in a secure environment that is away from where you'd like to remain anonymous.
2. Never upload sensitive files to third party services unless you are okay with the potential for eyes other than your own viewing these files.
Some privacy-enhancing technologies include:
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Linux: While there are a vast amount of linux distributions out there, varying in their security/privacy hardening out-of-the-box, almost all popular Linux distributions beat Windows and Mac OS when it comes to privacy.
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VPN: Even a VPN is not enough in some circumstances these days, however a VPN can help to hide your day to day internet usage from your ISP and protect your IP from services online by passing traffic through their servers. Look for a non-logging VPN with a reputation for privacy-first. A bad VPN would make using one redundant.
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Tor: If you need another layer of security, using Tor over VPN is a good way to enhance your anonymity.
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Firefox: Firefox is great because you can learn to harden it further than just in the settings page. There are also some browsers which have forked Firefox and have hardened them so that they're more privacy friendly out of the box. Privacy from tracking is dependent on your browser!
Avoid google and services using big tech to preserve your data and privacy further. It's a hit to convenience though there are almost always just-as-good solutions to google services that are much better for your privacy.