Hit the power button on your monitor, then turn the power supply switch on (on the back of the power supply) and then press your PC's power button.
Hit the power button on your monitor, then turn the power supply switch on (on the back of the power supply) and then press your PC's power button. If everything is working, the PC should turn on and run its POST (power-on self test). Since your operating system isn’t installed yet, you may get an error message about a missing boot drive, or you may get sent straight to the UEFI/BIOS.