So this is going to get long I think.

I bought a pc from best buy, yeah not the smartest move. The pc was <-- that.

I recently tried to upgrade the graphics. I got a gtx 770, an 850w power supply, and the haf 912 to put it all in. I ran in to a couple of problems when I was running all of the cables from the power supply. In the old case, with the old power supply it had a 4 pin power connecter to the left of the CPU socket. I wasn’t sure if this was a required connecter to run the pc, but I thought it was after fiddling around a bit I found that there was a 8 pin power connecter that came with the new power supply, and I also find that I could fit 4 of the 8 pins into the 4 pin connecter. 

Is that bad? Should I have not have done that? Should I just find a 4 pin connecter online or at a store?

But I went on to put everything together, get it all connected and proceed to power on the computer. It posted the hp start screen and it said to press esc for boot options. But when I tried to press esc it wouldn’t work. This is odd for a few reasons to me, one the hard drive I’m using is the same one that I was using before this and should still have an operating system on it, and two I know the usbs were working because my keyboard is backlight and the lights came on.

If you know what I did wrong or you could provide some tips to trouble shoot this some more, it would be greatly appreciated, also if you could answer my questions above that would be great as well

So depending on what psu it is you will have the 24pin atx, 8pin cpu (thats the one you want to plug into that 4pin) pcie cables - you'll need an 8 and a 6 pin for your 770.

ok now again depending on what psu you have look at the big sticker on the side, look at the 12v section. If there is mutilple columns labeled 12v1 12v2 etc. You will want to run a seperate main cable from the psu to the gpu (ie 1x 8pin 1x6pin) . Reasoning behind this is that if there is multiple rails when a high end card such as the 770 draws power at full load the psu needs to be able to push (feed) the card 42amps of 12v power into it. If say there is 2x 12v rails on the psu each at 31amps then a single rail wont be able to feed the gpu. That why one would need to run a cable from both rails.

Just make sure you never plug a 8pin pci cable into the cpu power (4 or 8pin socket)  to the left of the cpu. This will most likely result in a dead motherboard. Most psu vendors mark the cables anyway - atx, pcie etc.

Hope all this is of some help mate.

Thanks for your help, and so you know the power supply I have now is <--that

Also I did some looking and there was a 4+4 pin cable that I had tucked out of the way without knowing. also I tried putting the old graphics card in to the motherboard and then it worked so I think it may  be that I have a amd cpu and a nvidia graphics card with the 770, or maybe it’s just the motherboard. its the hp angelica. Does that make any sense to think that? And if it is that is there a way around it?

I know your problem. It's happened quite a lot to my family before. You bought a branded computer.

so what does that mean?

great psu you chose there mate.  very nice indeed.

You seem to be the only one giving me useable advice here. Do you have any idea why my computer seems to reject my new graphics card? because like I said I put the old card back in and it booted right in to windows but when I have my 770 in the computer it just boots to a screen that says press esc for start options, but then it won’t recognizing that I have a keyboard plugged in.

Since its a prebuilt HP system i suspect that secure boot is enabled;

Instructions for manually disabling secure boot:

1) Power down the system

2) Remove the gtx770

3) Boot the system using integrated graphics or you old gpu

4)Enter the BIOS - mash DEL, F2 or whatever your system wants

5) Set Secure Boot to disabled

6) If there is an option, set CSM (or compatibility or legacy mode) to enabled.

7) Save the new settings

8) Power down the system

9) Install your gtx770

10) The system should now boot

11) Install latest drivers - via nvidia website.

thank you for all of your help, but i did this and it still didnt work when i tryed with the 770.

i found the fix. it turns out that the bios was just old and out of date. so i updated the bios, and i then had the legacy support and the secure boot setings that i could disable and enable. thank you for all of your help i couldnt have goten this to work with out it.

Thats good news mate. Happy days.