Are you talking about marking a target or locking on to one?

In order to lock on to a target you are targeting, and thus have your camera focused on said target, you just have to push the left stick button, or L3 if you want to use Dual Shock notation.

If you're trying to mark a target for others to attack, you select your target, press the Square button using Dual Shock notation(or X if using a logitech F750 or 360 controller), select 'mark' from the mini menu, select an icon to use, then hit accept on your controller and that's it.