University students who are just starting their studies for four-year bachelor's degrees should generally avoid picking majors right away; instead, they should focus on diverse sets of core classes that might shed light on interests and aptitudes. Also, consulting with a career guidance counselor can provide insight on future job prospects or internships to gain valuable experience.
Certain situations may call for an early declaration, such as a complicated program that requires a lot of classes. STEM (science/technology/engineering/math) majors are fairly in-depth and require students to declare early to avoid staying at the university for longer than necessary. It's also possible to pick a double major if the student doesn't mind the extra workload or feels the two majors can complement one another.
When deciding on a major, it can be a good idea to talk to students or peer advisers who have experience in that major to get a feel for the difficulty of coursework. Signing up for a class or a seminar provides first-hand experience.
Also, it's possible to change majors, though doing so adds to the time spent at the university and the number of required courses. This is why it's good to take a diverse set of core classes -- they provide the groundwork for advanced courses.