If you have a laptop or netbook, there are probably some screen brightness controls on the keyboard. They usually have stylized "sun" icons, and are often on or above the function keys (e.g. F4). On some laptops, you must hold-down the Function (Fn) key and then press one of the brightness keys to change the screen brightness. For example, you might press FnF4 to decrease the brightness and FnF5 to increase it.

You can also change the brightness by clicking your name on the top panel, selecting System Settings and then opening the Screen preferences.

If you have a desktop computer, you should be able to use controls on the monitor to change the brightness of the screen. How you do this depends very much on what monitor you have - some have specific buttons to change brightness, whereas others have buttons which you use to navigate on-screen menus.

If you have increased the brightness to the maximum but it's still not bright enough, you may need to adjust the contrast or gamma settings of the screen instead. Not all screens allow you to change both of these settings, however.