Learn all you can in your first job. You will learn on the job, practical applications of things that were only covered briefly in school. Be curious about all of the different business processes going on around you. I know as a newer employee it is sometime very difficult to learn about what goes on in other departments, but try to learn and observe. In a startup you will need to do it all. You are studying marketing. You will also need basic knowledge in many other fields. You need to know about sales, accounting, management, operations, the whole scope of business. You don’t have to be an expert, but you at least need to know what some of the duties and goals of each department are. Otherwise, how can you manage and direct your staff.
Work on your business plan. Define your mission statement and goals. Develop a plan for success. What you will need, who you will need, and how much do you expect it to cost to accomplish, both in time and money.
In addition, develop relationships with people in other disciplines that may be able to help in your startup. Find like minded people, with similar motivations and passions for their work. You want self starters that are able to work independently.