Push the Power ON button. That is all there is to it.
Booting, short for bootstrapping, means to start your computer. The computer "pulls itself up by its boot straps" by starting from the small power button to a tiny program hard wired in the motherboard to the BIOS, (Basic Input Output System), program on a chip, to the BOOT Manager program on the device selected, and finally to the Large Operating System.
If you mean booting from a disk/device other than your main disk drive then you must set your BIOS the first screen that comes up, to look to boot on a different device. This is usually done by pressing the F2 key after the power on key to enter the BIOS setup program. There you can find the "Boot Order" Follow the instructions on the screen to set the device you want to boot from at the top of the list. The actual media, eg. CD/DVD, USB drive, etc. must be inplace in order to boot from it.