First time building new computer from scratch

[tnthinhn]

Me and my buddy had bought all the parts of the computer and put it together. Everything were new and the CPU is compatible with the motherboard. The system were fine but the monitor does not connect. At first we thought it were the cable or the monitor that went wrong but it wasn't. The motherboard seem fine and we input the video card also but it's still doesn't connect. What could have went wrong?

[GroundZero3]

Please list all of your system specs including your power supply

[tnthinhn]

AVC Socket 775 Aluminum/Copper Heat Sink Fan w/4-Pin conne (New)
Intel DG41TY Intel G41 Socket 775 mATX Motherboard w/DVI, Vi (New)
Cendyne 2GB DDR2 RAM 667MHz PC2-5300 240-Pin DIMM (New)
Samsung SpinPoint F3R HE103J 1 Terabyte 1TB SATA/300 7200RP (New)
Intel Pentium 4 531 3GHz 800MGz 1MB socket 775 CPU (Refurbished)
ASUS GeForce 8400GS 1GB DDR2 PCI Express PCIe DVI/VGA Low Pr (Refurbished)
Blue Star 650w 20+4-pin Blue LED Fan ATX Power Supply w/SATA (New)
What could have gone wrong?

[JPMiller]

Well the most obvious place you went wrong was buying an $18 650W power supply, or using it if it was included with the case.
Before you can troubleshoot ANYTHING, you have to replace that....
Truth is that setup only needs a GOOD 300W PSU.
I personally wouldn't buy anything less than a GOOD 400W for future upgrades, but that's all you technically need now...
Newegg.com - SPARKLE ATX-300PN-B204 300W ATX 12V 2.2 Power Supply - Power Supplies
This is the highest quality, affordable PSU you'll find, but it has little headroom for the future...
Since you obviously don't know the importance of a quality PSU, I suggest you read through our sticky...
Power Supply Info

[tnthinhn]

Thank you for the tips I'll try that !
What about that 650w what set should I put together?
Is that a even more powerful and better or it just depends on varieties?

[JPMiller]

Huh?

What about that 650w what set should I put together?
Is that a even more powerful and better or it just depends on varieties?

Your 650W is garbage and needs to be replaced before you can even begin to troubleshoot...
Forget wattage, it means nothing, you need a decent +12V amperage and a quality brand,... period...
If you read the sticky you'll understand...
I suggested the cheapest quality option that will run your rig stably, You can pay more and get higher wattage, but its a waste unless you'll upgrade within a year or so...

[tnthinhn]

Hey guys I order that PSU and it still doesn't work why ?
What should I do now?

[JEROY]

Did you try to use the integrated VGA instead of the Nvidia one ? ... Some monitors do not recognize Nvidia VGA's easily ..

[tnthinhn]

Well we try that too but it still doesn't work
Is there anything that we missed?

[JPMiller]

Well in general, refurbished parts are a great deal and usually very reliable... however some bad parts do fall through the cracks, I would try a different video card.
When you tested the VGA port on the motherboard, did you do it AFTER removing the dedicated card...
It wont be recognized if there is a video card plugged into the PCI-E slot.
Youll also have to test the CPU since that is refurbished as well.
Even though it wasnt the root of your issue, its impossible to reliably test other components if you have an unreliable power supply.

[tnthinhn]

It still doesn't work!
What to do?

[JPMiller]

So.... you tested the onboard VGA AFTER removing the video card...
Then tried a different video card, AND a different CPU?

then the next step would be pulling everything out of the case, and reassembling it outside the case on a nonconductive surface.
ONLY attach the CPU/HSF, 1 stick of memory, and use the onboard VGA.