Your computer may still chose to turn on using the integrated graphics as primary video card, so you'd need to have the video cable connected to motherboard video connectors to get some image.
In this case, you must go in bios and select primary video card to be the pci-e video card.
With SOME video cards and when using HDMI cable if my memory is correct, it can take a bit too long time for the video card to start up and detect a monitor, so the BIOS may think there's no monitor connected to the pci-e video card, and chooses to switch to using the integrated video card hoping there's a monitor connected to that one.
This can be solved by using a DisplayPort cable between the video card and the monitor, or maybe a DVI cable.
Also when you turn on a motherboard for the first time, it can take 30s to 1m for it to initialize fully and show some image. If you got scared every time and turned off the system seconds after booting without an image, wait up to a minute without any image.
May also want to press the button on the monitor to scan for signal (if it has such a button, or just turn it off and on)