All I want to do is post my Minecraft server on the main Minecraft Servers website but I don't really know what to do with the code they gave me. Please help?
The Message of the Day (motd) can be changed in the server.properties file. The line you're looking for should be at the bottom:
motd=A Minecraft Server. Just change that to whatever you want your message to be, though there seems to be a limit of 59 characters according to the wiki. You can also change the formatting using some standard formatting codes.
Go to your Minecraft server folder then open the server.properties file. If you scroll down at the bottom you should see something saying
motd=(your motd). Replace that with the code they gave you and run the server.
Near the bottom of
server.properties, you will see
motd=A Minecraft Server
A Minecraft Server to whatever you want.