I'm not too sure about healing the party leader, but for healing yourself with a modifier or your target without the following macro would work (just replace Healing Wave with your own healing spell):

/cast [mod:shift,target=player][help] Healing Wave

I think the party leader is either party 1 or party 2, but I've never used those much, so could be wrong. If the leader is party 1 then your macro would be this:

/cast [mod:shift,target=player][help][target=party1] Healing Wave

Macros read from left to right and will execute the first correct command they come to, so if you hold shift down that will evaluate true and you'll heal yourself. If you've not got shift down it will skip to the next part and heal your target if it is friendly (that's the [help] bit), if you have no target, or it isn't friendly then it will try to heal the party1 player (hopefully the group leader, but I'm not sure).

If you're using this macro on an alt then you'll need some way to target your intended heal recipient or clear the target if you want to make sure that the leader will be healed.