Step 3:

Test the Computer

Reassemble the Tower
Put the case together
Plug in the monitor cable
Plug in the keyboard
Plug in the mouse
Plug in the power chord
NOTE: Everything should be plugged-in and ready to use. There are four (4) essential things to plug-in: Monitor, keyboard, mouse, and power. There can be more things, such as speakers, microphone, modem, network cable, USB hub, digital camera, and the like, however, these items are not necessary for initial testing.
Install Operating System (Windows, MAC, Linux, etc.) Software
Power-on the tower
Open an optical drive (CD-ROM drive)
Insert an operating system CD-ROM
Follow instructions and prompts