Secure Boot

Secure Boot is a new feature found in Windows 8/8.1/10 PCs that prevents "unauthorized" operating systems (such as GNU/Linux distros) from booting. All computers that have Secure Boot as a feature are all UEFI. UEFI is a new firmware that replaces the traditional standard BIOS firmware, and is basically on every new computer from 2012-present. UEFI is backwards compatible with the old traditional BIOS firmware also, in which it's called Legacy. You can set your computer to boot as a Legacy computer if you have a UEFI BIOS, to boot to operating systems such as Windows XP. Secure Boot also sucks if you're trying to downgrade to Windows 7, since Windows 7 wasn't made when UEFI was becoming mainstream.

I wanna disable it!

To disable Secure Boot on a Desktop computer, do the following.

1. Restart your computer.
2. Press F1/DEL/whatever to enter your BIOS when the screen shows up on boot (this is called your POST screen)
3. You're in the UEFI BIOS now!
4. You need to find Secure Boot. It should be in either a Boot tab, Authentication tab, or a Security tab.
5. After disabling, press F10/whatever to save changes and exit.
6. Congrats! Secure Boot is now disabled. Now, install Gentoo!

To disable Secure Boot on a Laptop, it is a bit more complicated, and AFAIK, this isn't the only method. You can also search your laptop's BIOS key, and hold it to get to the BIOS (e.g ASUS laptops, hold F2 while starting up).

1. From the start menu, or the lock screen, open the power menu (with the power icon) and hover over Restart.
2. Hold down Shift on your keyboard, and then press Restart.
3. Let go of Shift when it says "Please wait".
4. You're now in the Advanced Options Menu!
5. This is a new feature in Windows 8/8.1/10 since you can't hold a regular key to get to things such as Safe Mode.
6. Select "Troubleshoot".
7. Select "Advanced options".
8. Select "UEFI Firmware Settings".
9. Select "Restart".
10. Congrats! You're in the UEFI BIOS! Now, find Secure Boot.
11. It should be in either a Boot tab, Authentication tab, or Security tab.
12. Disable it.
13. Press F10/whatever to save changes and exit.
14. Congrats! Secure Boot is now disabled. Now, install Gentoo!