When you Boot the System, it starts on default Runlevel specified in /etc/inittab:
Id:?:initdefault:
When System Successfully boot, it will ask for username and password. But you don't know the root's password. To change the root password you need to boot the system into single user mode. You can pass the kernel arguments from the boot loader.
1. Restart the System.
2. You will get the boot loader GRUB screen.
3. Press a and type 1 or s for single mode ro root=LABEL=/ rhgb queit s
4. System will boot on Single User mode.
5. Use passwd command to change.
6. Press ctrl+d