As Sturok said, if you are on PC all you have to do is click on your tank and hit T to switch to their target.

For PS3 or PC with gamepad, you will need to make the macro:

/target <tt>

This will do the same thing as hitting T on the keyboard. Just stick it on your action bar by hitting Square on the macro in the macro list, then "Set to Hotbar". After that all you have to do is target your tank (remember, up and down on the D-pad targets your party members) and hit your macro to change to his target.