To script a server, you'll need to learn the Squirrel programming language. After, you can use the functions, events, and server settings found here to make your own custom scripts:
http://www.liberty-unleashed.co.uk/LUWikiThe server itself comes with a basic, generic gamemode script that you can take a look at for an example. There are also multiple files in the server's root directory that control things such as manual blip placements, character spawn selection classes, and the like. These files can generally be viewed and edited using a standard text file editor like Notepad, or a similar third party application.