Most boards will auto default to dedicated video cards.
You will have to download the latest drivers from Nvidia.
I would like to help you better understand the 12 volt rail on your power supply.
Computer power supplies have one or more 12 volt rails. With single rail power supplies you have a single number for the 12 volt rail, with a multi rail setup you have more then one.
You can NOT add the rails on most power supplies and most list a combined rating.
This image may or may not be your power supply. It is a multi rail unit with 18 amps on the 12 volt rail. This is a bit low for a 300 watt unit, but should still run a 750ti.
Red is the 2 rails green is the combined and the math required to convert wattage to amps.
Here is an example of a 300 watt power supply I have my one of my systems.