I actually think it's a bit premature to start setting up macros before learning the skills and when/where to use them. At this point in the learning curve you're better off using the skills directly and learning how to target directly.
And yes, as a healer, a lot of your attention is going to be on your team's HP bars. Making sure you can keep everyone alive (especially the tank) is your first priority, and when you're first starting out it's fine to make it your only priority. Becoming proficient as a healer is just about keeping everyone alive. Going beyond basic proficiency into being highly skilled involves knowing when to focus on that and when to switch over to dealing some damage. You'll see threads on the forums talking about how healers need to be able to deal damage as well, but that's mainly a higher-level concern. When you're just starting out, it's ok to just focus on the basics. For a healer, the basics are seeing who needs a cure and curing them.
As to your concern about being able to switch back to your party members quickly enough when needed:
Are you using the controller scheme where you hold the R2 and/or L2 down while also pressing a face button in order to perform your cross-bar skills? (I think that's the default pattern, IIRC.) If so, it frees up face buttons without the R2 or L2 to be used for other features, including targeting. All of the following only applies with that controller scheme (which, as the one I use, is the only one I'm really familiar with):
The up/down arrow buttons will cycle just between your party members (in the order they appear in the party window). That's usually a pretty fast way to get to any of them. Selecting someone with the up/down buttons and then activating a skill will soft-target them for that skill, meaning that one skill is directed to them, but then your target returns to whatever it had been. Selecting someone with the up/down buttons and then pressing X to confirm that selection will hard-target them, meaning they'll remain the target of all your skills until you select someone/something else.
Similarly to the up/down to select party members, holding L1 while pressing up/down will scroll through the (already engaged) enemies that appear in the enemy list. Use the same pattern of following it with X if you want it to become your new hard target, or not if you want to soft target it for only a single action.
The left and right buttons will cycle targets among things in front of you, according to your current targeting filters. I believe the initial default targeting filter when you begin the game is "All" meaning it will cycle between all nearby players, mobs, NPCs, objects, companions, pets, and minions, sometimes making it quite difficult to select the particular one you want. (This is a bigger issue in crowded FATEs where all might be present at once than it is in dungeons, but it's still worth setting up some other options.) Luckily, while the default isn't always great, it is configurable.
If you go into Character Configuration in the Control Settings section, Filters tab, there are a couple ways to customize your targeting filters. At the top it has a section for selecting which types to target while your weapon is drawn and which while it's sheathed. Below that is a section for using up to four filters based on the controller's face buttons. Only enable one of the two types, as enabling both will make them interfere with each other. I prefer the face button selection: You can set it so that, for instance, L1 + [square] would put you in "Friends" targeting mode, L1 + [circle] will put you in "Enemies" targeting mode, and L1 + [triangle] will put you in the default "All" targeting mode, or you can also set up custom modes by choosing the individual checkboxes for what to include in your filter. Once you've set that up, then you can alternate your targeting mode by L1 and a face button. The left and right arrow buttons will only cycle between targets of that type until you use L1 and a different face button to change your targeting mode to something else.
Incidentally, in dungeons where the only friendly entities are the other players in my party, I usually keep my targeting filter set to "Enemies" mode, since up/down already handles my party members anyway. I use the "Friends" targeting mode during quests where I want to heal an NPC, or in open world content where I may want to heal players who aren't in my party.