Ubuntu – Grub menu not showing up at boot

My computer shipped with Windows 8 and later I upgraded to Windows 8.1 Right now I'm trying to get Windows 8.1 to dual boot with Ubuntu 12.04, but the Grub menu doesn't show up at all. My computer just boots straight to Windows. I searched for this online and the only answer I get is to edit /etc/default/grub, but I don't even know what this is and where to find this. I also can't use a bootable CD, only a USB. I really need step-by-step answers that aren't confusing like all the other ones I've found so far.

EDIT: I just rebooted and now the Grub menu does show up but when I try to boot into Ubuntu, it says a file is missing. Windows boots normally though. I have disabled secure boot and fast startup.

    Windows failed to start. A recent hardware change or software change might be the cause.
    To fix the problem:
    1. Insert your Windows installation disc and restart your computer.
    2. Choose your language settings, and then click "Next."
    3. Click "Repair your computer"
    If you do not have this disc, contact your computer administrator or computer manufacturer for assistance.
    File: \NST\AutoNeoGrub0.mbr
    Status: 0xc000007b
    Info: The application or operating system couldn't be loaded because a required file is missing or contains errors.

I'm not sure if that file was corrupted while installing Ubuntu or it's missing.