1. Wait for the tank to grab all around enmity

Especially if your tank is a Paladin. About 5-10 seconds. A good cue to start attacking is when the healer starts to heal.

2. Focus fire on tank's primary target

Good tanks will always mark. If you come across ones who don't mark. It means they are that bad-ass or just bad. Usually the latter. In that case, after you have waited, click on tank to see its current target, then target that target.

Tanks have to build all around enmity in the initial pull, so the first creature they attack may not be the primary target. This is also why we wait.