Motherboard won't read graphics card

I'm building my first pc and am currently undergoing a test build. I set everything up and plugged my monitor into the hdmi slot in my video card. Powered on and nothing happened. So I decided to plug the hdmi cable directly into my motherboard. When I powered on with this setup I got bios, with some options to do things like "load default optimization, then boot". So why can't I plug in through my graphics card? Someone suggested to me that I need to install drivers and/or disable my mb's onboard graphics, but in a build video I saw, the guy basically did exactly what I am doing and got some information on his screen. I know my card is powered on because the fans are working.

Update: I only have one pci express plug plugged into my graphics card, but my card has 2 pci-e sockets. Do i need to plug into both of them?

Here are my specs:

**CPU** | [Intel Core i5-3570K 3.4GHz Quad-Core Processor](http://pcpartpicker.com/part/intel-cpu-bx80637i53570k) | $189.99 @ Microcenter
**CPU Cooler** | [Cooler Master Hyper 212 Plus 76.8 CFM Sleeve Bearing CPU Cooler](http://pcpartpicker.com/part/cooler-master-cpu-cooler-rrb10212pg1) | $19.99 @ Newegg
**Motherboard** | [Gigabyte GA-Z77X-UD3H ATX LGA1155 Motherboard](http://pcpartpicker.com/part/gigabyte-motherboard-gaz77xud3h) | $139.99 @ Amazon
**Memory** | [G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory](http://pcpartpicker.com/part/gskill-memory-f312800cl8d8gbxm) | $72.99 @ Newegg
**Storage** | [Western Digital Caviar Black 1TB 3.5" 7200RPM Internal Hard Drive](http://pcpartpicker.com/part/western-digital-internal-hard-drive-wd1002faex) | $84.98 @ Outlet PC
**Video Card** | [Sapphire Radeon HD 7870 XT 2GB Video Card](http://pcpartpicker.com/part/sapphire-video-card-111992020g) | $254.29 @ SuperBiiz
**Case** | [Fractal Design Define R4 (Black Pearl) ATX Mid Tower Case](http://pcpartpicker.com/part/fractal-design-case-fdcadefr4bl) | $79.99 @ Amazon
**Case Fan** | [Corsair Air Series AF120 Quiet Edition (2-Pack) 39.9 CFM 120mm Fans](http://pcpartpicker.com/part/corsair-case-fan-co9050002ww) | $27.09 @ NCIX US
**Power Supply** | [Cooler Master Silent Pro M2 720W 80 PLUS Bronze Certified ATX12V Power Supply](http://pcpartpicker.com/part/cooler-master-power-supply-rs720spm2d3us) | $130.98 @ Newegg
**Optical Drive** | [Asus DRW-24B1ST/BLK/B/AS DVD/CD Writer](http://pcpartpicker.com/part/asus-optical-drive-drw24b1stblkbas) | $18.99 @ Amazon