Needing some help with positioning & awareness

[Rilifane]

Zoom out your camera to always have a better overview over what's going on. Wouldn't recommend aerial view though.

Put your party list in a size that's comfortable for you either to the left or right off centre. You need the centre free of UI elements to better see what's going on directly around you but if your party list is too far at the side you'll lose sight of it easily.
Put all other information somewhere clumped together where it's easy for you to see. Some people tend to always look at the top of their screen, some to the right etc.; I tend to look down so that's where my important stuff is.
Things like your own buffs, debuffs and target's debuffs. The more your eyes need to scurry across the screen the harder it will be to keep track of everything.
This is my current healer UI on WHM:
https://i.imgur.com/XfANg5Y.png

I have my list of buffs directly under my list of the target's debuffs and both are arranged over my job gauge and hotbars with my 'more important' skills that I use on a regular basis. So see my buffs, my remaining dot time, my job gauge and my cooldowns at a glance.
On the right side above my hotbar are my own debuffs.

I strongly recommend targeting via party list. As soon as people start moving around or a lot of effects are on the screen you will have trouble targeting them by avatar alone.
You can also focus target your tank and bind 'target focus target' to a hotkey as crutch while you get used to healing via party list but try to get rid of this habit soon. Focus targeting the boss is generally prefered.

How to organize your skills is a bit difficult. There are a myriad of ways and some depend on your hardware. I'd try to organize skills by use.
I have my dps skills, my ST heals and my AoE heals clumped together and all easy to reach while my buffs are on keys that are more or less comfortable and easy for me to reach, depending on how fast and often I use them.
E.g. Surecast is a bit harder to reach for me but I use it 1-2 times per fight at most and I always know in advance when I have to use it so I could even click it(though I'm an avid defender of hotkeying everything and not clicking).

Once you found a way that works for you, keep it the same on every healer. Meaning 'same' skill (WHM Cure II = AST Benefic II = SCH Adlo) on the same key. Arrange the rest around it.