Step 1: Acquire Computer
You can purchase a PC at a computer retail store in your area or on a website. The site I would go to to buy a new PC would probably be www.newegg.com
Step 2: Locate Power Button
Step 3: Press the Power Button
Holding down the power button when it is on will force it to shit down immediately. This can be useful when your computer freezes although you can lose unsaved work and risk corrupting your entire hard drive (that last one is extremely rare).
Step 4: Troubleshoot Problems
1) Your computer is not plugged into power
2) Your power supply isn't turned on
3) Your computers bios settings need to be reset
4) Power button not connected to motherboard
After verifying that your computer is plugged into power and the power supply is turned on but your computer still won't turn on, unplug your computer from all power whatsoever, open it up, remove the small round CMOS battery, and put it back in. Now plug your computer back into power and turn your power supply back on. If it still won't turn on, the power button may not be plugged into the motherboard. At this point, I would recommend taking your computer to its manufacturer and having them diagnose it.