Setting a BIOS/UEFI password might help (But doesn't always). If that does not work on your computer, you could try replacing the windows bootloader with grub as the default bootloader in a weird windows menu. (I don't remember exactly how I did it. You should search for something like that on the internet.)
On some devices windows puts itself first in the boot order. I've seen this on some HP devices.
Setting a BIOS/UEFI password might help (But doesn't always). If that does not work on your computer, you could try replacing the windows bootloader with grub as the default bootloader in a weird windows menu. (I don't remember exactly how I did it. You should search for something like that on the internet.)