To make things a little more newbie-friendly
This subforum contains most of the mods and maps; there are also two stickies with a list of them, but they might not be always up to date. Your downloads will be either .zip or .rar files: you can use the basic windows unpackager for .zip files, but you'll need WinRar for the .rar.
Once you have the file, you have to extract mods into the "mods" subfolder of your dominions installation, and maps in the "maps" subfolder. Make sure that you actually extract the files directly in the mods/maps folder,without creating an additional subfolder, or the game won't recognize it.
Once you're done, you should find the maps available when starting a new game, and you can enable the various mods by going into options/mods inside the game