[Michael B]

If you don't have enough knowledge to open a local-hosted SA-MP server, simply download JernejL's map editor from here, and place it inside your game's folder. However, if you would like something more advanced, you can download the latest server pack for Windows from here, add Pottus Texture Studio, which can be found here, and run the server.
If you would like a tutorial about running a server, check this out.