My custom built computer will turn on, but no video will go to the screen?

First YOU need to benchtest, pull everything out and connect the hardware i'm going to list outside of pc case. take out motherboard,cpu plus heatsink, ram(only one stick),vga card(or if the motherboard has onboard video use it first,keyboard,psu. All of the components connect them outsiden the connect monitor now start the system. What do YOU see or hear? Black screen? Or beeps? Make sure to apply thermal paste correct and don't reuse the old thermal paste.