Usually, unless told otherwise, you will DPS the mob that the tank is focusing on. Depending on the pull, it will either be a verbal/party chat designation of target order or the tank might put symbols on each mob and designate a kill order.
If there are a couple of normal mobs in the pull as well as some strongs/elites, generally you will kill the normals first, even if they aren't being tanked because they go down fast. However, if those mobs are a bit away from the tank, you may stay near the tank as melee assuming you have ranged DPS to pick off the outliers. If your group is all ranged, then take out the normals as fast as possible then switch to the tank's target.
If the tank isn't designating a kill order, ask for one. Kill orders make pulls a lot easier and keep things under control. If you run with a regular group or guild mates, this is when something like Mumble or Vent can be really helpful. Also, assuming you have a regular FP group or guild group that you run with, you'll eventually get used to how everyone works together and you will no longer need explicit kill orders.