Now, before I show you some possible builds, I’d like to stress that you should try your own thing instead of copying one of these.
Pick some talents you want, fiddle around with the talent builder on the wow site and see what you come up with. The builds I offer are nothing more than suggestions.
Affliction build

It is very difficult to decide on a build that you will be completely satisfied with in the end. Most people end up resetting their talents multiple times, and start doubting their choices later down the road.

This build is very well suited for a levelling, as well as raiding. The extra mana you have access to through Dark Pact allows you to keep up your damage output much longer.
Dark Pact is not quite as useful in PvP, but it has its moments there too.
PvE-wise; For this particular build, having Curse of Exhaustion and Dark Pact allows you to indefinately fear, snare, and dot a target. (Using a pet to replenish your mana) Given a free space without likely adds, you can take down elites at or just above your level easily.
This is doable with just fear too, if more difficult, but CoEx gives you the added option of kiting unfearable (but snarable) targets.
Having access to Siphon Life and Improved Howl of Terror allows you to DoT multiple mobs and eassily fear them off to watch them die. Adds caused in the process can be dealt with by casting those same dots some more. Siphon, when ticking away on multiple targets that are not hitting you, will grant you sizable health regeneration. Depending on what you're fighting, at a certain amount of spell damage your Siphon will outheal a mob's damage.

Destruction build

Destruction is the tree of choice if you wish to deal more damage, faster.
You are also out of mana faster, but that’s the tradeoff.
An excellent build for PvP, but it does not perform poorly in PvE environments either. Excellent at taking down trash mobs, but much more reliant on outside heals to keep up the damage in longer fights than Affliction and Demonology are.

Demonology build

Demonology is a powerful and very versatile talent tree. It mainly focuses on improving your pets, and your own power through the use (or sacrifice) of pets. This tree focuses on survivability, and you will be able to withstand a great deal more damage then with any other build.
A fully demonology specced warlock is extremely resilient thanks to Soul Link. The 20% damage transferred to your pet makes a big difference. You can imagine what it does for you in both PvE and PvP situations.
Focusing on this tree makes the decision on which pet to use (or sacrifice) doubly important due to the added buffs they bring through Master Demonologist and Demonic Sacrifice.
Long fight with lots of AE damage coming up? You may want to sacrifice a pet. A boss with less AE damage? Use that Felguard for added damage.
Also, remember you can have a sacrifice buff going and enslave a demon.
Every situation has its own approach, and you can try different things every time to see which works best. Demonology gives you more options than you would otherwise have.

If you have a hard time making choices, pick another tree :P
The main drawback is that while many of the effects from the Demonic Sacrifice and Master Demonologist talents look awesome, you can only use one at a time. It makes the choice of pet (or no pet) much more important, but it does allow you to be effective in a broader range of situations than before.
Affliction / Destruction build

If you want to be an affliction warlock, but desire the bigger crits from Ruin, you will have to forego getting Unstable Affliction.
Pro’s of this build as opposed to a full affliction build:
- Bigger critical nukes kill. The ability to take a large chunk of hitpoints away in an instant is a really big plus in PvP a regular affliction lock doesn’t have.
- The +% chance to crit on items becomes twice as effective.
Cons:

- The bigger crits serve less purpose in PvE. While they do provide extra damage, they are also putting you at a bigger risk of overaggroing a mob. Having to slown down your damage after landing a couple of crits can be annoying. Although that time can be spent regaining mana.
-Unstable Affliction provides more net damage than Ruin until you reach an astronomically high amount of spell crit, practically unachievable with the gear in the game.
In short: Bigger crits, a little less damage on non-trash fights.
Carrying on with some less common builds;
Soul Link / Ruin build

The big crits from ruin, the resilience of Demonology and instant Corruption. What more could you ask? Well, you're giving up on Felguard, which is a pretty kickass pet. Still, this build isn't entirely insane and worthy of consideration.

Super Imp

A rather silly build, more of a joke than a serious option. Still, the build is surprisingly damaging when you have a change to utilize it properly. It has some major cons though.
The biggest one being that your imp can take basically no damage before keeling over. Any fights with AE's are off limits for the imp. Whenever its possible though, your imp will be dealing considerable damage with those little bolts. He runs out of mana easily, hence 3/3 Mana Feed. You can keep him nuking indefinately with that. Consider giving it a try next time you want something different.

Dark Pact / Master Demonology build

If you choose for maxing Mana Feed, you will have a virtually endless supply of mana. Every Life Tap will be worth double the mana, as you can drain whatever your pet gets through Mana Feed away again with Dark Pact. All that mana is not something you will normally need, but this hybrid build does allow you to use a reasonably nice felhunter, succubus or voidwalker without having to worry about their slower mana regeneration. An interesting build, if nothing else. Haven't tried it myself yet.

Demonic Sacrifice / Shadow and Flame build

This build maximizes your fire damage with a sacrificed imp but give up a lot in other areas. Most of the talents you are forced to get to reach Demonic Sacrifice in Demonology will be practically useless to you. You lack instant Corruption and Shadowfury and have to choose between Nether Protection, Backlash and Soul Leech. Its not my favorite build and I'm putting it late in the list for that reason.
That covers the most commonly and some uncommonly used builds, disregarding small placement differences in certain talents. Where one would rather get Improved healthstone over Fel Intellect or make a change elsewhere, the overall idea of the builds tends to remain the same.
I hope these example builds gave you some insight on what to do with your talents, good luck on making a decision!