You'll want to start with the [target=<something>] command for your macro. I'm not sure of the exact specifics, but...
Code:
/cast [nomod] Righteous Defense
/cast [mod, target=targettarget] Hand of Reckoning; [mod, harm] Hand of Reckoning
Since a macro can't check a cooldown of an ability, you could run it in this manner, where the baseline macro would essentially be Righteous Defense. If you hit a mod key (Alt, Shift, or Ctrl) you'll cast Hand of Reckoning, and it will determine the cast based on whether your current target is friendly (HoR on target's target) or hostile (cast on target).
I'm not sure of the specifics, but I'd suggest learning the macro syntax. It can actually be fun to mess around with.
Also, since the topic is "Paladin Macro", this won't be horribly off-topic...
Code:
#showtooltip
/cast [nomounted,flyable,nomod] <your favorite flying mount>
/cast [nomounted,noflyable,nomod] <your favorite land mount>
/cast [mounted,nomod] !Crusader Aura
/cast [mod, spec:1] !Devotion Aura; [mod, spec:2] !Retribution Aura
/dismount [mounted,mod]
In a flying enabled area, hitting the macro will have you mount up. Tapping it again will activate Crusader Aura. Hitting a mod key (Alt, Ctrl, or Shift) and the macro will dismount you and activate an aura based on your spec (for me, spec:1 is Prot and spec:2 is Ret).
NOTE: This won't work well in Azeroth right now, since technically all areas are flyable, but nobody has their flight license. So all you'll see is your grayed-out flying mount icon until we're allowed to train our old-world flying skill.