Pick a first step you can undo.
You do not need to move everything this weekend.
I want to see NixOS.
Boot the official graphical live image or use a VM. Look around before deciding to install. Trying a live environment does not require replacing the operating system on your disk.
Upstream references (1)
I want to try the configuration model.
Use a NixOS VM. Make one change, build it and check what happened. Keep the example small enough that you can explain each line.
I want Nix, not a new operating system.
Follow the official installation instructions for your platform. Then try a temporary shell or a development environment for one project.
Before installing
Back up files you would miss and check that you can restore them. Confirm which disk the installer will use. Test essential applications and hardware. Keep a way to get back online and boot recovery media.
Read the installation guide before changing partitions. The examples on this site are not a complete installation procedure.
Keep the first configuration yours.
Add a few packages. Enable one service you understand. Rebuild. Read the error when something goes wrong.
Once that feels familiar, look at modules, Home Manager or flakes. A working understanding is worth more than a large configuration you cannot maintain.