Computer I just built will turn on but nothing on monitor.?

Sometimes its the most simplest things that throw us after puttingtogether a computer of our own. It seems like you've covered the basics already though. Make sure that everything is seated properly and fully. Next check out the video. Have you tried hooking up your monitor to the onboard video? If you get video from that you may want to move on to checking the videocard next. As i said before, make sure it is seated properly and that power is supplied to it from the PSU. If that comes back bad after the video checks good on the onboard video for the motherboard, your videocard is bad. If the onboard didn't give you a video then yor motherboard is bad.