Depends on the device. There are lots of stuff out there.
And, it also depends on what is your goal. Do you want to be untraceable or just secure? If you just want to be secure, then a compatible device with strong encryption with a password that has to be entered every time you lock the device along with a messaging app that is End to end encrypted will be sufficient.
Computers are the easiest devices to lock down properly. Phones and tablets on the other hand have security built-in but you can't really tune it to your needs. I'd suggest full disk encryption on computers for starters, I recommend Veracrypt.
If you want to be untraceable, then it gets more complex. You cannot let your IP leak at all. If it does, then they can start using the legal framework to subpoena ISPs and so on to give your info/location etc. So, a VPN would be one of the things you should use, and this should have the necessary features like a kill-switch etc. And remember, the VPN company will have access to your real IP so if the authorities crack down on them, you might be in trouble. So, make sure the laws of the VPN company and the hosting company & the server (important to check all!) are in jurisdictions where there are laws to prevent disclosure. I think Switzerland may be one of them, but you should double check. Mullvad is also a great VPN, they have 0 KYC and you can buy access by mailing money or via crypto.
All in all, it's not easy if you're not an IT person. It's best to just go for the tried and tested methods. And remember, this stuff is almost always free except VPNs etc. But being completely untraceable is hard. All ot takes is one mistake and its all shattered.
If you have more specific questions, please ask away.
