While on a Live server you can type /testcopy to have that toon copied over to the Test server, otherwise you can simply create a new toon on Test. Either way, when you bring up Launchpad, click where it says Live and change Select Game Version to Test:
If you're planning to play on both Live and Test, it's a good idea to make a separate installation folder for each. That avoids massive repatching to switch between versions.