Leveling 10-20

Upon reaching level 10, you get a quest for your Voidwalker demon and start to earn talent points each level. The Voidwalker should be your pet of choice for a good many levels as it serves as a relatively decent tank for you while you nuke. Talent points can be used to give you new spells or abilities or to make current ones better. These next ten levels should be about setting up your talent build and building a good understanding on how the class plays.

Talent Points
All three talent trees available for the Warlock are very nice. Affliction focuses on your Damage Over Time spells and debuffing abilities, making them better. Demonology focuses on making your demons more powerful and giving you some extra survivability abilities. Destruction greatly strengthens your nuking ability. They all have their pros and cons and usefulnesses. Destruction makes your Shadowbolts more powerful and mobs die faster, but you consume far more mana and will need more downtime to replenish. Demonology has a lot of relatively useless talents that you must get in order to reach the really good talents. Affliction has less burst DPS potential, but it is more mana efficient and requires little to no downtime.

Any of these you could level successfully as, but Destruction is clearly the less desirable of them due to its mana consumption. Save this for End Game. Demonology and Affliction are both good leveling builds, but early Affliction talents are clearly better. Thus I propose that the best way to level is to start out Affliction and then when you have enough talent points to get the good stuff, respec to Demonology.

The first 10 points should be used as such:

1. Improved Corruption (1/5)
   
2. Improved Corruption (2/5)
   
3. Improved Corruption (3/5)
4. Improved Corruption (4/5)
   
5. Improved Corruption (5/5)
6. Improved Life Tap (1/2)
7. Improved Life Tap (2/2)
8. Improved Drain Soul (1/2)
9. Improved Drain Soul (2/2)
10. Soul Siphon (1/2)

You could switch Soul Siphon and Improved Drain Soul around if you like; there won't be too much difference either way. What this group of talents are doing is giving you another instant DoT in Corruption to go along with Curse of Agony. Improved Life Tap will allow you to gain more mana per Life Tap. Improved Drain Soul lets you gain mana back if you Drain Soul while a mob is dying, which is a good practice to get used to anyway, since that is the only way you can get Soul Stones, which you need. And Soul Siphon makes your Drain Life return more health as you have more DoTs on a mob.

Tactics
Your ideal fighting tactic now should be something like this: Send Voidwalker in. Curse of Agony -> (Immolate) -> Corruption -> Shadow Bolt -> Life Tap -> Shadow Bolt. If mobs die fast, you don't need Immolate; if they last longer though, use Immolate. CoA should always be used first because it lasts longer and does more damage at the end, therefore you want it to tick all the way through. Keep Shadow Bolting and Life Taping till mob is almost dead, then quickly Drain Soul right before it is about to die to get some mana back as well as a Soul Stone.

Now you will find that your VW is not that great of a tank as you would like. At first it will do a fine job as you are just laying your DoTs on the mob and not doing any burst damage. But as soon as you start Shadow Bolt nuking, you will pull aggro. There are several different ways to deal with this. One is to just deal with it and keep nuking, hoping the mob dies soon, or fear it to give you time for another Sbolt or two. Another thing you could do is just wait a bit between DoTing and nuking so that your VW can build more threat. The third method is to Drain Tank.

Drain Tanking is another one of those unique Warlock things. Since Locks value stamina so much and generally have higher health than other casters and have the ability to Drain Life, they can take a greater beating and outlast mobs. Basically you DoT up the mob, Life Tap, and then Drain Life till it's dead, Life Tapping any time your HP is full or a lot higher than your mana. Drain Life does not do as much damage as a Shadow Bolt would, but if a mob is on you and preventing you from casting that SBolt and damaging you anyway, Drain Life comes out on top in that situation. Drain Tanking gets a lot better in your 20's as you get some other talents, but you shouldn't be needing it as much right now anyway as your VW will be tanking the mob most of the time. But it is a good tactic to use when you are in trouble, or just to get used to it for later on.

Gear
I didn't comment on gear before because, well it wasn't important at that early of a level. It still isn't terribly important now either, though you do want to at least focus on needed attributes. As a Warlock, you want Stamina and lots of it. There used to not be too any items with +spell damage early on, but there is more now, so that should also be desired. And since you are a mana user, you also need Intellect, though not as much since you can just Life Tap. Therefore you should prioritize Stamina > Spell Damage > Intellect. I prioritize Stamina over Spell Damage for now since as you are soloing you are likely to not get healed and want the extra HP to take a beating (useful for drain tanking) and need it for Life Tap anyway. "Of the Eagle" gear is very good to get. Spirit is very useless for a Warlock since they do not consume as much mana as other casters and can Life Tap it back.

There really isn't any specific gear to note at this level since you will be replacing it so quickly. Just look for Stamina and Spell Damage gear, with a bit of Intellect on the side.

Abilities
There isn't many noteworthy abilities during these levels, mostly just newer ranks of the one's you already have. At lvl 10 you do get Create Healthstone, which is useful. You should always have a Healthstone on you just in case you need it. Drain Life at lvl 14 is valuable, of course. Create Soulstone at 18 might be handy if you ever feel you are in a tough spot and want to be able to resurrect. Curse of Recklessness, Health Funnel, and Searing Pain are all insignificant at this level.

Additional Notes
Since summoning the VW, Healthstones and Soulstones require Soul Shards, you better get used to acquiring them via Drain Soul. Not only that, but you should accumulate Shards so that you already have one ready to be used when you need one. It will depend on your bag space, but you should always have at least 5 when soloing, 10+ if you group a lot. (Your group will want some Healthstones too!)

Get in the habit of using Life Tap only when you have used enough mana to get the benefit of the full Life Tap or when your HP is full. Alternatively you could alternate between Life Tap and Drain Life and, in effect, keep your HP and Mana nearly full at all times.

I was thinking about adding a Demonology (or Destruction) section for those of you who want to go that way right away, but I will save that for an alternative leveling guide later on. Again these early Affliction talents are way better than anything Demo or Destro offers early on, so it's much better to just start out Affliction. The only thing of note for solo leveling early on for Demonology is Improved Voidwalker, but it's not as effective as you might think it would be. If you really like the Tank-and-Spank playstyle, you can get by respeccing at lvl 17 so that you get the benefits of the awesome Affliction talents till you can get Imp VW in full.

EDIT:
I completely forgot about wands. An alternative strategy is to DoT up the mob while your VW is tanking and then just wanding it down while your mana regens. This is perfectly acceptable and actually is probably the desired strategy for right now. Drain Tanking isn't too strong yet (and you can get a wand before you even get Drain Life) so you might want to hold off on that for now.