This is just off the top of my head, so please read this as a general intro and not a precise instruction to follow.
1. Use an OS that does not "dial home" with info about what you do with your system, i.e. use Linux. If you need to use MS-Win or Mac software, there are software solutions in Linux that will let you do that, such as Wine for ms-win or Darling(?) for mac, or a virtual machine.
2. Learn how to create secure passwords, and how to use a password manager. You must be ready to memorize at least two longer passwords fully: one for the full disk encryption, and one for your local password manager software.
If you choose the diceware method for your memorized passwords, it really has to be long enough, say 6 words plus preferably a few random non-letter characters and numbers.
3. Full disk encryption, this encrypts the whole system drive including the operating system. Also, any extra drive that this computer will connect to should also be encrypted.
4. VPN that you pay for, if it is 'free' they might choose to make money by selling something else, such as data collected on you. Choose a VPN provider that has gone through a recent third-party security audit from an established firm. Mullvad and ProtonVPN usually gets good reviews, and I know that Mullvad have software that make it easier to set up and manage, esp on Linux.
5. Online services that you use must be safe, and have close to zero connection to you. For example. If you need to use email on this computer, or need to give an email adress to register software that you will use on this computer, or if you from this computer will log in to forums that asks you to give an emailadress to make an account - you must create a new email adress (or several) that have no traceable connection to you. Protonmail is one of the usual recommendations here, but you need another email to register with them. That other email can be a disposable one, but you might have to try out several disposable ones to ge tone that Proton accepts. Don't give Proton an email that is connected to you, or even worse a phone number - this breaks your anonymity.
Payments online are really really hard to make anonymous. The only way I've read about that seems credible is Monero, a crypto currency. Crypto currencys are a world onto itself, and one has to read up a bit to be certain of how to use them without exposing ones identity and at the same time without loosing ones money.
Does all this seem a bit too much? Sure. But right now one unlucky guy in Australia is facing a possible long prison term over rendered underage characters. And several other countries have legislation that could be interpreted to convict people for material shared on sites like this and others.
Also, this was just a quick throw-together, not detailed instructions to follow. If you want to read more there's a lot written about it. For instance privacyguides[dot]org has some stuff that gives a basic orientation. I don't know if the forums over at wildersecurity[dot]com is still good, but it used to be ok. There are some subreddits too, but one has to know to sift the good from the bad there.