Dot, dot, dot.
Warlocks are the most powerful area of effect damage class in the game, focusing on the elements of disease and poison to bring their enemies to the point of collapse. They work particularly well with Defilers, who can debuff their targets' resistance to those elements significantly. They can also buff their allies and debuff their enemies, and generally do well in putting out lots of damage over longer periods of time thanks to their extensive damage over time spells. They're quite mana efficient, being one of the casters with the greatest fight longevity, up there in terms of sustained damage with the best scouts. Their most important statistics are intelligence, power pool and power regeneration, as they have little need for avoidance or stamina - although it can help to have a gear set with some of those statistics for when you'll be exclusively putting out area of effect damage.
Warlocks have many instant damage spells, but they should be generally used only when you have all your damage over time spells up and running. They're far less efficient in terms of how much damage they do over the entire fight, and only help to improve your damage when you already have those up and going. With a competent tank, you shouldn't have many threat issues, but as you're leveling and playing with people still learning, you might run into some problems unless you control your threat output carefully. Boon of the Dark is an excellent tank buff that gives back power to the target every time they're hit in melee. This helps to increase their overall threat, which in turn allows you to produce more damage. Dark Siphoning is another great spell that drains power from an enemy and returns it to the entire group. Curse of Null debuffs the target's strength and intelligence. Warlocks can also remove arcane debuffs and dispel beneficial spells from their target, adding to their utility.
Similar to the Wizard, the Warlock can use the Tap Void spell to transfer some of their health to power. The Netheros spell summons a strong pet that will attack the Warlock's target for a short period of time. If you find yourself pulling a lot of threat from the tank, it's a good idea to slip the threat reducing spells Nullmail and Concussive into your rotation to stop that from happening. Don't be shy about suggesting that you use your area of effect spells - they're some of your most powerful. Encourage your tank to use their group taunting abilities, and be sure to back off if you start to pull threat.