I want to become a 153A and have no clue where to start. Any advice?

Two ways prior to joining the Army:
1) Talk to a recruiter
2) Read the information on going into the Warrant Officer Flight Training* as a civilian .. and then talk to a recruiter.

If you enlist in the Army and want to transition while your are in-service, then you can review the process on the USAREC site*.

Now, to your question about what makes a great packet (i.e., what makes you most likely to be chosen). That's going to vary by year - not what the board believes is most important, but how far down will they reach. Think of each year as being graded on a bell curve, and the board does a "whole of candidate" evaluation, so even if you were low in one area (i.e., SIFT score of 41 or a mediocre ACFT, etc.) you can compensate if you 'shine' in another.

Here are a few things that the various feedback and board results (from when they actually gave feedback and showed the selects and non-selects) show:
● ACFT: Get in the best possible shape you can. This is one of the few things you can actually directly control. Average APFT (not ACFT) score from the last WOFT results I found (FY2020) was a 262.
● SIFT score: While the Army says a minimum of 40 is acceptable, the reality is that you want to do that best you possibly can get (scores range from 20 to 80). If you pass (40 or better), then that's what you'll have .. period. There is no retaking the SIFT unless you fail (and in that case, you have to wait 180-days). There are MANY sites on the web that will give advice on preparing for the SIFT. Average SIFT score was a 54.
● Civilian education grades: Higher GPA is better. Study for those college courses you are taking. Average GPA for those selected in WOFT was 2.9.
● Applicant Essay ("Why do you want" .. "Why are you ...") - spend a good amount of time writing it in order to make it as concise and meaningful as possible. Write a draft, put it away for a few days, take it out, rewrite it. Do that a few times. Show it to others both to give feedback on content (i.e, WHAT you are saying) as well as syntax (i.e., HOW you are saying it).
● Letters of recommendation (LOR) - Three to six of them. You should actually have the majority of your packet done (SIFT, physical, resume, etc.), especially if you're going to cold-call an Aviation Warrant for a LOR.

Discuss the LORs with those who would be writing them (you'd be amazed at the number of senior individuals who have never written one). They should focus on skills, character, capabilities, and military experience (if your including them in a civilian WOFT application, then they will obviously not speak to military experience). What the board will be looking for in the LORs are items addressing "Are you going to be able to handle complex/technical tasks"? Do you have leadership potential? What's your character like? What is outstanding about YOU that sets you apart?

If you're starting to get the impression that everything is important, then you are correct. As I said previously, the board will do a "whole of candidate" assessment of your packet. The more in the packet showing that you consistently hold yourself to a high standard (regardless of what an established standard is), the better.
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* USAREC site page - joining WOFT as a civilian
* 153A while in-service - https://recruiting.army.mil/ISO/AWOR/153A/
* USAREC Sample Warrant Officer application/guide - https://recruiting.army.mil/Portals/15/Documents/WO/Sample_Packet_as of 22 MAY 2018.pdf

There are Points of Contact at the bottom of the page. There is a "How Do I Apply" hyperlink as well

https://recruiting.army.mil/ISO/AWOR/153A/