Instructions
Reach Level 10 with your warlock by completing quest chains in your starter zone. Non-player characters (NPCs) that have a quest available will appear with a yellow question mark over their head and on your mini-map. Talk to them to accept the quests and refer to your quest journal to find out what the quest requires.
Go to your warlock trainer and learn to summon your voidwalker demon. Use him as a tank in battles.
Proceed to the Level 10-20 area, which is the area that borders the starting zone. For example, Blood Elves should go to the Ghostlands; humans should go to Westfall.
Complete more quests until you reach Level 15, then use the Dungeon Tool to join instances with groups as a damage-dealer. Continue questing while are waiting for a group in the instance queue to make productive use of downtime.
Return to your warlock trainer once you have reached Level 20 and you will be able to learn to summon the succubus.