When you enter the Academy you provisionally select the division that you'd like to be in, for example, if you thought you would like to be a Tactical officer upon graduation then you would enter provisionally as Tactical. In first and second year you take courses from across all the different divisions; this is the equivalent of basic training.
Note: if you've already got a degree from a civilian university and you're going to go into the same line of work in Starfleet, then all you'd need to do is first and second year at the Technical Academy to cover your basic training!
Congratulations on making it through your first year! It's still basic training time, but here you get to build upon your existing learning. This year is all about working out what division you're going to go into in third year, so use this time to learn what subjects you enjoy and prefer!
At the end of second year you have to choose the division that you want to specialize into for third year onwards. This can be the division you provisionally signed up for, or it could be totally different! This year, most of your courses will be for your own division. You can see that some cross over however; and if you already have a specific career in mind for after graduation, you can cater your courses around this. Keep in mind that most career-specific training would happen on top of these courses, and these courses would instead act as additional training. For example, a prospective Health and Safety Inspector would receive specialized training for their desired career.
In fourth year it's time for the advanced subjects, and these modules usually last a semester each. You can only do one or two per semester depending on the nature of the module. For example, if you take a practical placement or cadet cruise then you will not be doing another module alongside it. Study advanced modules, write dissertations, or gain practical experience in preparation for your graduation. Congratulations Cadet - you're almost there… Unless you're studying to be a medical doctor. Unfortunately, you've got longer to go!