If it's a laptop there are often two keys bearing 'sunshine' and 'darkness' symbols. Pressing those keys adjusts the brightness of your screen. (With some laptops the relevant keys are among the 'F' keys, at the top of the keyboard. With others the brightness symbols appear, usually in a different colour, on two of the 'normal' keys, meaning that you have to press 'Fn' simultaneously to activate them.
If it's not a laptop then the actual monitor should have brightness settings (via buttons at the side or front) on it.Alternatively, if your PC has got a separate video card in it (rather than 'onboard' video) you can usually adjust the colour and brightness by right-clicking on your desktop and selecting Properties > Settings > Advanced > Color (or something similar; it varies).