Your question is a common one, so I'll try to address how it works:
Your first shot to apply has typically been after winter break as a plebe. You'll have the opportunity to express your ranked preferences for specific slots. Pay attention to those emails from your chain of command and TAC-NCO!!! And bulletin boards!
Your chances of getting what you want as a rising yuk are largely influenced by your overall rank (Academic/Military/Physical) with a heavy emphasis on Military. (can vary by years and TAC/TAC-NCO's)
So your CBT (beast) performance and 1st plebe semester will largely determine what you get. This surprises some cadets who are running under the radar. There appears to be a structured approach to selection, but it's also clear that TAC-NCO's have a good bit of influence. Can fight for exceptions, newly opened slots, etc. And they tend to do that based on cadet performance.
There are other gates:
- Have to have passing APFT, and will have to take one before you leave. So can't be borderline.
- Can't be on profile (injured), "conditioned" (under performing academically, military, or physical), or in trouble for specific actions. Many injuries occur in CBT, and your physical condition can impact that. So physical condition matters more than most realize. Ex: many cadets who have problems with pullups have problems with CBT confidence courses and get injured. I know of a dozen or more cadets this has happened to.
The number of slots has been trending down, and it's now very hard to do two MIAD's.
Airborne and Air Assault have become quite competitive as a rising yuk, typically just 2-3 sessions of each are available because you also have CFT that summer.
It will vary by year and number of slots, but typically the top third to half of the class will get ABN or AASLT if they want it. (Only workable slots for about a third typically, but not all want or can take them)
It's also worth thinking ahead... if you plan to branch infantry some select Air Assault over Airborne as USMA infantry grads are normally automatically sent to ABN if they do not have it.
But there are tradeoffs as well... if infantry w/o ABN it will delay the report to your first unit, etc. And there are pro's and con's of attending Ranger School w/o ABN yet.
But if you are branching non-infantry, this may be your best shot to get your choice in the current military budget situation.
If you are not ranked high enough to be competitive there are other MIAD options, as well as AIAD's (study abroad, etc).
Lately the emphasis has been on getting each cadet at least one *IAD. You will have later chances, but it gets harder, as for upperclassmen firsties without an IAD get first priority after plebes. (more complicated than that, but there is a priority)
It can vary by year and individual session, but the general theme has been that ABN is more fun, AASLT is typically much harder. Two years ago cadet AASLT experience was a huge smokefest. Things changed, last year they smoked less but the testing was much stricter. So YMMV. ABN is generally more popular and is sometimes seen as a proxy for class rank. (Not always true)
This is general flow. Any individual experience can vary, even between companies/reg or year to year. The gates and process are fairly consistent of late, but can change at any time.
so the short version is, if you want your pick, be an exceptional cadet in CBT & 1st semester. That's in your control. Balanced across the three towers. The opportunities will come.