Ok so what you need to do is touch 2 pins of your motherboard with the tip of a screwdriver. This acts like the power button that tells the computer to power on. The pins that need to be touched are likely labeled pwr on or power switch. You can find where these are on your motherboard by looking in your manual. You will need to plug the power button from your case in to those pins later.
I am assuming you have the motherboard plugged into the psu by the 24 pin connector and the 4 or 8 pin cpu connector. Im also assuming your cpu, ram and gpu are already installed and connected. You may need to plug your gpu into the psu aswell