Targeting someone by mousing over: /cast [@mouseover] Holy Shock
Addons that are useful: WeakAuras, Gladius
There are alot of ways to target. Some people just click the nameplates and heal like that - even high rated players. Others will make macros like /target party1 and /target party2 and bind them easy. I'd pick shift+mousewheel up and shift+mousewheel down, but that's my way of doing it. Others again just make macros for every heal. Lets use holy shock as an example.
You have /cast [@player] Holy shock, which casts it on you. /cast [@party1] Holy Shock and /cast [@party2] Holy Shock casts it on party1 and party2 respectively. Others again play with the same dudes always and just stick their names in there instead of party1 and 2.
Probably the absolutely fastest way is using this last system, but it's not needed if you don't like it.