Halflife mods - Adding a map to your server.

Locate your servers main halflife folder.
Change to your mod folder.
If your server is counter strike then you will change to cstrike, Day of Defeat will be dod etc.
Look for the folder called maps.
Upload your maps here and restart your server. Change to your map by typing map <mapname>

A few notes.

Make sure the map you are uploading is for that game.
If your map won't work it may be corrupt. Download it again and then re-upload it.