How do I reboot my PC back to first day brought

What do you have, some have Hidden Partitions.

If your system came with a restore partition or disk you can consult your owners manual for the restore instructions. If you were to make the recovery disk and have not (HP) you can do so if your computer is still booting and running.
Most PC Makers use some form of restore software that is not part of Windows to allow the reinstalation of their Shovel Ware (extra programs added to your computer by the manufacture to receive payment from the publisher for their install) this is annoying but it dose in some cases keep the cost of a computer lower.

Rebooting is the process of powering down your computer and powering it back up. What you want to do is restore the computer to factory settings.

The only way to do that is if you have a Recovery Disk or partition. If you have the Recovery disk, just pop it in and reboot then follow the instructions. If you don't have the disk, you can try pressing a key during boot to access the recovery partition. If you have neither, you are a bit stuck.

Why do you want to do this?