There's a few different strategies to leveling Warlocks. I always preferred to mix Affliction with Demon, getting some of the best talents from both. This is probably the best way to level, but you can try anything you like.
With 9 in Affliction: 3/5 Suppression, 2/2 Improved Drain Soul, 2/2 Life Tap
With 11 in Demo: 2/2 Healthstone, 3/3 Voidwalker, 1/1 Fel Domination
This can be done as early as level 29, with Amplify Curse 2 levels away.
http://www.wowprovider.com/Old.aspx?talent=11226005_9This is a mana efficient build with a buffed tank pet. Without the talent, the Voidwalker will struggle to keep aggro. Fel Domination is less important but convenient.
If you do want to go up Destruction, I'd suggest doing it after getting these talents I recommended. Nukes aren't as mana efficient in general as dots, and the tree has no healing or mana talents in it past that 5% cost discount. That kind of makes up for Suppression, but isn't as point efficient nor affects your most useful spells(Fear, Drain Life, Drain Soul, Death Coil). I should point out that Conflag is basically a wasted spell to use on mobs. You may as well just let the Immolate run its course. Shadowburn, which doesn't eat your Immolate buff, is at 11 points in the tree. Conflag is mana efficient, but only if used towards the end of the Immolate CD. As I said, you could just let the debuff run its course without bothering with the spell at all. I do want to play Destruction again, but it'll be at max level for BGs, lol, not leveling up.