In many schools, your second and third years are largely completely at your discretion. However, if you participate in Law Review or other publication, and/or Moot Court (or Mock Trial), a great deal of your time will be dedicated to those activities.
That said, I do believe that it is useful for law students who have a general idea of their future practice areas to take certain courses. For most people, the first choice is pretty basic- whether to litigate or engage in a transactional practice ("transactional" as in corporate and business transactions, such as contracts, mergers and acquisitions and the like).
And even if your plans change, in my opinion, it is almost always of benefit to litigators to know something about transactions, and for transactional lawyers to understand the basics of litigation. The two are inetricably interwoven: transactional disputes lead to litigation, and in that litigation, the litigators must understand the substance of the transaction in dispute.
So, if you plan to litigate, you should definitely plan on taking Evidence (usually in your second year), which usually deals with the Federal Rules of Evidence (the rules describing the admissibility and handling of evidence in litigation). You probably have already taken Civil Procedure your first year (which deals with the mechanics of litigation), another extremely helpful course for future litigators. In addition, you should consider any courses or legal clinics which will give you faux or actual experience in the area. For most people, the hardest part of litigation is simply understanding the landscape- who does what. These courses will help you to "hit the ground running" after you start that first legal job. There are numerous other courses which are of benefit, however, if you understand the basic procedures involved in a lawsuit, you will be well on your way to a successful litigation career.
As an aside, there are certainly substantive specialty courses you should consider, especially if you know the specialty that you will practice in. For example, if you have always wanted to practice in family matters, you should definitely take your school's course on Family Law. However, generally these courses can only give you a very broad overview; the reality is that you will master the law and practice area only by actually practicing it. So, don't be alarmed if you find yourself working for a firm that wants you to do legal work in an area with which you are unfamiliar- the resources of the firm, its other lawyers and your own experience will help you to master the area.
Corporate transactional work, on the other hand, is a bit more technical. For example, tax law and securities law are both very statutorily-based and require a great deal of technical knowledge. I think it would be very difficult to begin a career in securities law without having taken the '33 and'34 Act courses while in law school. Likewise for tax and some of the other more specialized areas in the transactional discipline. So, if you plan to engage in a transactional career, I would advise you to seriously consider taking the core "business" law courses- tax, securities law, secured transactions, commercial paper, and the like.
Regardless, the point of all of this is to not stress any of it too much. I also recommend that you simply take courses that interest you. Talk to 3Ls and professors, or any other people you may know who have gone there before you. It is almost axiomatic in law school that you take the professor, not the course (meaning that a bad professor can make a great subject pretty ungratifying, and vice versa). Do some due diligence and perhaps even take a few risks. If you make a mistake and realize it early in the semester, you can drop the course if necessary.
Law school is a general educational discipline. It is more concerned with teaching you how to think than doling out the "black letter" law. As one of my professors said, "We don't worry about teaching you the black letter law; you take Barbri for that." However, at the same time, you are likely investing a great deal in this educational experience. You will only do it once; make the very most of it you can while you are there.