Have you downloaded the files needed to create you scripts? If not here is a link http://www.sa-mp.com/download.php
You need your gamemodes/filterscripts etcetera before you can host a server.
If you already have these then, go to http://portforward.com/ Choose your router, then click SA-MP Server and follow the instructions, it should tell you how to get a static IP address, which is what is best if you are home hosting. I would reccommend you look into buying a 24/7 hosting plan from any server hosters, just look online.