Sit the motherboard on the table on the static bag it came with.
Fit the CPU only.
If it has a speaker connector plug in the speaker. Temporarily remove it from the case if you don’t have one spare. Or it may have an onboard speaker check the manual for that.
Connect the power supply sockets. The 24 pin ATX power socket AND the PCI-EX additional 12 Volt feed.
Do not connect anything else, there is no need. Mouse keyboard monitor ect.
Then using something metal like a screwdriver to short the pins to soft boot the motherboard.
Even with only the CPU and the motherboard you should hear some post boot bleeps. It should sound like this.
Beep, pause, beep, pause, beep, pause. Continued.......
Power off the power supply. Don’t keep it running to long without the fan.
But a few seconds is ok to test.
If you hear the bleeps then fit one stick of RAM. And retry.
If you have a built in graphics card then you should hear . a single beep.
If there is no built in graphics card then you will hear. Beep beep beep, pause beep.
If you didn’t hear any beeps. After the first process, then it would be a faulty motherboard and or CPU. Recheck you have connected all power cables to the motherboard.