Being an x-wow noobie ill help you out
1st- You will need a Soulshard, which can be found by using the spell "drain soul" on a character that gives you experience, drain soul must still be cast while he is dieing to create a soulshard.
2nd- If you havent already, you will need to visit your trainer and get his quest, killing the voidwalker gives you the spell to summon him.
If you have, you can just use the soulshard and click "SUmmon Voidwalker". Simple as that