UIs are very personal things and there is a ton of information out there. A good strategy is to stick with the default UI for a bit and then when you find yourself going “Man, I really wish the UI could do X” you start looking around to see if such an addon exists.
While you’ll probably want to eventually graduate to use addons to make your UI work best for you, here are some tips to make the default UI work a lot better as a healer.
When in edit mode, make sure that “Party Frames” is checked in the main window so you can edit them. (If you don’t see a list of things to check click on the expand options arrow.)
Right click on the party frames (they’ll be on the left side of your screen by default on top overlayed with the raid frames) and check the “Use Raid-Style Party Frames” checkbox. Adjust their size and position to your liking.
Now go into the Options menu. Go to Gameplay>Interface and scroll to the bottom. You’ll see a bunch more raid frame options you can play with. (And, since you are using raid style party frames, they’ll apply to those as well!)
Next, most healers prefer using mouseover, click casting, or some combination of the two. (They are basically the same thing, but mouseover works with keybinds and click casting works with mouse buttons.) These will allow you to heal someone without changing your target.
For Click Casting using the default UI, go to Options>Gameplay>Keybindings and click the click casting button to bring up it’s interface.
For mouseover macros, type /macro
into chat to bring up the interface. Here’s a basic mouseover macro:
#showtooltip
/cast [@mouseover,help,nodead][help,nodead][@player] Healing Spell Name
You can then drag the macro to your action bars like a spell from your spellbook.
What that will do is if your mouse is hovering over a something that is friendly and alive, it will cast the spell on them, regardless of your target. (You can either be hovering over their party/raid frame or their actual character model out in the world.) If you don’t have your mouse over such a target, it will heal on your target, if they are friendly. If you don’t have a friendly target either, it will cast it on yourself.
The next thing you’ll want to do is test all of this out in an environment that is low stress. Enter the Proving Grounds! For an Evoker, you’ll have to go to them manually (unless you’ve got a garrison). You want to talk to Trial Master Rotun in Kun-Lai Summit in Pandaria to enter them. (Other classes can talk to a class trainer to enter them, but I don’t think that Evokers have class trainers in the main cities.)
Once in, you can talk to him to start up some trials with NPCs where you can test out your UI setup and practice healing a bit.