Before you do anything, go to the card manufacturers website, look up the card, and get the latest drivers for it. Save the driver to a USB stick. (look this up on youtube if you must).
1) Unplug the PC's power, and hold down the power button to discharge any leftover power in the system. Touch the PC case to discharge any static you may have built up on your body.
2) Put the card into the motherboard carefully and secure it so it doesn't fall out and damage the motherboard.
3) Plug in the HDMI cable to a monitor and make sure you close the PC case. Put power back into PC.
4) boot the PC, install the drivers from the USB stick, and then restart the system. You should be good to go.