Securing Your Keys & Seeds
If you don't own your private keys, you don't own your assets. You should NEVER ever share your private key with anyone.
Do NOT Share Private Keys
Sharing your private key (or keys) is like giving a pile of cash to a stranger and saying, "Here, hold my money while I just go hang out at the local pub." No, actually, that's a bad analogy: A stranger might be honest and not run away with your money. A better way to emphasize this point, which will resonate with people who have little faith in humanity, is to point out that giving other people your private key is about as wise as throwing all of your cash into the ocean, off the back of the Titanic.
Don't give a private key to people who are teaching you how to work with cryptoassets. Don't give a private key to online support staff or exchanges. Don't give a private key to your brother-in-law, or girlfriend, or bank manager. The only way you can keep a private key secure is to make sure that you never share it with anyone. We're not saying that you shouldn't keep a secure backup somewhere! But if you have any significant amount of funds associated with a private key, then the only backup you might want to consider is something along the lines of encrypting it into a digital file that is placed on a USB key locked in a safety deposit box at a bank. You can never be too paranoid.
Do NOT Share Seeds or Seed Phrases
Same thing, same risks, same warnings. The same comments about keeping private keys to yourself also apply to seed phrases, and to the seeds that they generate. If a seed or a seed phrase can unlock your wallet, you need to make sure that nobody else ever has access to this information.
Generating a seed online is as bad as sharing a seed. Why? Because a lot of online seed generators are malicious, and designed by hackers. So even though you might not realize it, you may be sharing your seed with a criminal. Always generate seeds offline, and only with resources endorsed by the official developers of a project.
Do NOT Share Pass Phrases
Are you starting to sense a theme yet? If you use some sort of pass phrase to access a cryptoasset wallet, you should never share that, not even with friends and family. Sharing a pass phrase to a wallet can lead to unexpected loss of funds if someone decides to steal your assets.
Final Thoughts
Hopefully the fact that this page is one of the shortest on this site, and gets right to the point, doesn't minimize the importance of the message. When investing in cryptoassets, you are your own bank. Nobody else is looking after your interests. Never forget this lesson, and read through the computer security page and the scams and ponzi schemes page to make yourself as aware as possible of potential weak links in your personal security protocols.