Gigabyte GA-78LMT-USB3 Motherboard
Gigabyte AMD R9 380 GPU
AMD FX 8350 CPU
This is my first computer build idk whats wrong.
I did the power on self test and it'll show the bios for maybe 2 seconds and then it'll go to the disk boot failure screen.
If i plug in the power to the GPU the screen never shows anything
First, do not run the system with the power disconnected for the video card, you can damage the video card and possibly the motherboard. Second, you should have someone that built computers before look over your work to make sure everything is connected properly.
And upgrading the power supply as above for a power hungry system with that CPU and video card is also a good idea. If you used a lower power system with nVidia and Intel parts you would be Ok with the PSU you got, with the 380 and the FX 8350 you need a better power supply.
You also need a better motherboard for the FX 8350, read this http://www.tomshardware.com/answers/id-1896886/motherboard-amd-8350.html
it has some info about stability of the system with that CPU on cheap boards.
About the boot failure, did you install Windows on the hard drive?