if you're still insecure the best way to do it is to focus-target the tank at all times and use the arrows to temporarily hit other things without focusing
I play healer 95% of the time and I find WHM to be the easiest. There are a ton of "oh shit" buttons that are basically always available, lots of oGCDs.
I personally find WHM and SGE to be the most fun because with WHM I can basically solo heal anything, even on current content that's supposed to have two healers. On SGE I get to DPS but the problem is if the tank dips too low it's hard to get them back up.
I miss my many oh shit buttons while playing SGE lol. But WHM is really good just for building confidence in healing and branching out to other jobs if you want.
L3 can be assigned Assist! Otherwise you can macro your DPS or heals (pick the one you normally DON'T focus on swapping targets for) with the <tt> macro so they'll be cast on your target's target. (you'll still have to select with the dpad if you want to attack/heal someone else)
and then you get really mad when you're below the 70 range and you don't have your burst party heal