First you have to uninstall the onboard drivers.
Watch the screen on booting and it will tell you how.Most are Delete.
You can't disable it till you monitor is connected to the new card.
Unplug all Power connections,
Insert the card,connect your monitor to it .Just finger tight.
Then first go to bios to disable onboard before booting to Windows.
So be sure you know how and what key.
Then use F10 to save and exit bios and continue to Windows.
Your video card should still have the previous monitor settings.
It also helps to read your Motherboard manual,so this is the best i can explain this.