Requirement: Prestige Class Trainer. In Warj'Arune, you must unlock the 1st level of this prestige class by finding the appropriate trainer. This could be someone else who has the class, an ancient tome full of wisdom, an outsider with relevant abilities / knowledge, or via a number of other means.
Rule Exception: Summoning Undead. Shadowdancers are able to summon a Shadow without suffering an alignment shift towards Evil. They are pretty much the only class able to do this, to reflect the rule that the Shadowdancer's summon shares the caster's alignment. This only applies to the specific class ability, a Shadowdancer with levels of another spellcasting class that summons a Shadow or other Undead, using the other class's abilities, will suffer the usual alignment shift.
Note that although this is an exception to the standard Warj'Arune rules (specifically: summoning / creating undead is an inherently Evil act), most PCs / NPCs you run into will have no idea that your Shadow is any different from any other one…