You can't, you can't start on Nostalrius. You can't get to play on the, epic, amazing and unique Nostalrius realms. Since Activision Blizzard, Inc. Successfully, backed up by questionable laws, forced Nostalrius to shut their servers down.
You can however, subscribe to the forum and help us with keeping the community going.
And please sign this petition https://www.change.org/p/mike-morhaime-legacy-server-among-world-of-warcraft-community