If you're doing AOP 17 then it won't be that long until you're out of initial training - the expectation is you'll have passed Fleet Board within a year.
You join a specialisation, essentially choosing a specific role, each has a presentation of Defence Connect and during AOP you may get visits and talks about them, and I'm sure the reserve staff will try and get some recruiting in. Factors to consider include when their major training opportunities occur and how frequently and any bottlenecks in the process, whether you want a sea going role or not, whether you like the idea of being on a battle staff, and mobilisation opportunities, frequency and how long it takes to be potentially mobilised. Whilst it is possible to switch specialisations this is definitely an important decision to make - once you're a midshipman speak to the officers at your unit to find out the pros and cons of each, and perhaps try to make an acquaint weekend or two. A visit to a Triton Warrior weekend is an opportunity to meet with several specialisations.
At the same time you undertake Phase 2 training - the Junior Officer's Leadership Course (JOLC1) and the Divisional Officer's Course (DO) - you have to do these before undertaking anything more than weekend training with your specialisation. There are RNR versions of both courses - which DO's course you do probably doesn't really matter. I would massively recommend the RNR version of JOLC1, it is one of the best courses I've ever attended and the time spent on how to be a junior officer in unit is invaluable and not something you'll get with the RN course.
Phase 3 is getting stuck into becoming minimally useful in your specialisation, usually a 2 week training course followed by participating in a major exercise like Joint Warrior. This is the moment you'll suddenly realise it is all very real, that you are a commissioned officer being entrusted with a modicum of responsibility and interacting with regulars from the RN and NATO forces - hopefully you'll thrive.
There is also unit life which varies massively by unit and the size of their wardroom. Units with fewer officers tend to entrust significant responsibility on junior officers, especially upper yardies or those with a bit more life experience. You'll start picking up unit jobs, being responsible for the development of ratings and being pinged to organise various things. Do learn to manage your commitment, don't assume that just because 24 days is bounty that we magically plan to only ever give you 24 days worth of work - your specialisation and unit will both quite happily give you more than 24 days of stuff to do!
Your unit will have a YOTO (Young Officer Training Officer) whose job is to get you post AOP through all the above.
