I'm new to this..... Where do I place music files for others to be able to download from my computer

It depends on the Gnutella client - you might want to ask this question again in the forum specific to the client you're using.

But usually under "Options", "Settings" or "Preferences" menu you will be able to select which directories you'd like to share. In your case, you'd want to share the download directory (ie., "C:\My Downloads").