There are a number of things you can do to put yourself in the best position possible for a career in fashion.
First- which area in fashion do you want to work in? Design, sales, marketing, advertising, buying, pattern making; etc? Fashion is a wide and complex industry with many parts. Before determining what you should do to get there; figure out where you want to go.
Based on your goal- the next step is to declare a major that will allow you to be prepared for that role. Example: If you want to be a Fashion Designer, you want to have a major or courses in fashion design (of course), pattern-making, textiles, auto-cad, graphics, and other arts. If you want to be in on the business side of things- majors or courses in marketing, advertising, corporate communications, finance, and management would be best.
Third, as a student- you want to make sure you have at least one internship under your belt by the time you graduate. Whether a short-term summer internship, or a internship that is long-term throughout the year- find one that is inline with where you want to be. If you're lucky, you may find an internship that pays. However, what should matter in the quality of the experience and the company.
Find a part-time job in the fashion world. Even a job a Sales Associate at a store in your local mall will provide you with first-hand experience in the industry that can build your experience from the ground up, and show where you want to be. Nothing beats learning first hand.
Lastly, join student organizations. Fashion and arts clubs is where you want to be. If one at your school doesn't exist, make your own! Employers love candidates who take on initiative and leadership roles.