The standard method to style data in GeoServer is by using Styled Layer Descriptor files, more commonly referred to as SLD files. SLD is a mark-up language based on XML and offers a lot of flexibility and power when it comes to styling geospatial data. However, with this power and flexibility comes complexity. Tackling SLD files in a standard text or XML editor is not for the faint of heart and is prone to errors. Invariably, this leads to a trial and error approach to style the data, as you continually go back and forth, tweaking your SLD file to get the styling just right. Therefore, it is best to use a visual tool to design your styles and have this tool generate an SLD file for you.

Going into the specifics of creating an SLD file is outside the scope of this book. There are, however, several places that contain very good information about SLD files. The best source of information (this should be your first stop) is the GeoServer documentation...