You should always use the resolution the manufacturer recommends for your monitor, especially for working or browsing the web. This is usually the highest resolution available.
(If you like to have buttons or text appear bigger or smaller, you should use the scaling options provided by your operating system.)
Modern operating systems are able to detect your ideal monitor resolution automatically. If you haven't manually changed it, and you aren't missing video drivers, there's a solid chance that your resolution is already correct.
If you're uncertain, please check the links below for your operating system.
Apple has guides for changing your resolutions here.