Teach the child the basic hardware elements of a computer. Use simplistic language to explain the functions of the computer. You don't need to get into any complex details about processors, graphics and RAM; just familiarize them with the monitor, the main tower or desktop case, the mouse, keyboard and the CD/DVD drive. Explain the basic functions of each component. For example, you could explain that the case contains the motherboard, which is the computer's brain.
Show the child how to switch on the computer. Point out the power button to them, and let them press it to switch the computer on. As the computer boots up, explain that they don't need to touch anything until the operating system loads up.
Help the child get used to using the mouse. Point the cursor out to her on screen, and then demonstrate that whichever direction you move the mouse, the cursor follows. Explain to her that she only need to press the left mouse button to select things. Introduce her to double-clicking, but explain that she only needs to do this to select the desktop icons. Let her practice with the mouse independently for while.
Teach the child how to access the basic games included on most computers. Kids love games, so they are the best way to help kids gain an understanding of computer operation. As you show them how to bring up the games, explain the steps along the way. For example, if you are using Windows, when you open the "Start" menu, explain that you can use the same menu to open most programs on the computer.
Introduce your child to the Internet. Again, use online games and learning websites to keep them interested in what they are learning to do. Websites such as Nick Jr., Play Kids Games and The Kidz Page are full of free games for kids to play. Choose games such as Kai Lan's Super Snow Dragon on Nick Jr. and the Maze Race game on The Kidz Page, which do not require reading, for younger children. Chat-rooms can be fun for older children, but there are inherent risks. See Resources for information about online security for children.
Let your child play by himself. After you have taught the basics, leave your child to learn a little on his own. Kids are like sponges for new information, and they will soon learn how to operate the interfaces of the games and websites you have shown them.
Stay close to your child when they use the computer. You should check in on them occasionally to make sure they aren't doing something they shouldn't be, and you also need to be on hand for any problems they have. The problems they encounter are unlikely to be very complicated, and helping them out when they are stuck can help them learn more of the details that underpin computer usage.