I am trying to install a new motherboard, but the system powers down a second or two after I turn it on. The fans spin briefly then stop. No beep tone.
Nothing else is connected right now: Just the motherboard connected to the power supply via the power cable, the front panel power switch connected to the motherboard, and the system fan connected to the motherboard
At a minimum, you need a power supply, cpu, its cooler, ram and either on-board video or a video card on the motherboard to get past the power on self test (POST).
It also really helps to hook up a keyboard, mouse and monitor in order to tell what's going on and do something about it.
You will also need some kind of storage, (HDD or SSD), to put the operating system on and some way to get it there.