Everyone here is wrong on the most important thing.
The first most important thing to learn is:
1.) How to research properly, self-training, retraining for retention, and figuring out what is bullsh!* and what isn't. (AKA: a website telling you that super heat should always be 13° delta sounds good but is very wrong.)
Then I would say:
3.) Air balancing.
I find there is more problems with electricity and air flow then refrigeration.
If you're too "open" minded, your brains will fall out.
Artificial intelligence is no match for natural stupidity.