First, check to see if your power supply has an additional 4 pin connector, and if your motherboard has an additional 4 pin connector port too, which is most likely somewhere on the left of the CPU.

If so, that needs to be plugged in because it powers the video card.

If that isn't it, does the motherboard you bought have an on board video card built into it? Basically, another port to plug into the monitor? If so, then that is your default video card, and you need to change it to your other one.

The only way I really know of doing this, is to use the video card that came with your motherboard at first (hook the monitor up to it), then if it is a brand new custom built computer, install your operating system and set it up. After all that is done, and it is running, then shut down the computer, install the Nvidia video card into the slot on the motherboard and restart your computer.

If you are using windows, when you get to the desktop after restarting it, the new hardware wizard should detect the video card.

Use the installation CD, or the hardware wizard, to install the video card. Once you do, shut down the computer, plug the monitor into the NEW video card this time, then restart it, and it should work.