Where you'll start. On the account track, entry level is account manager. Other agencies might call that position assistant account executive. Account managers are usually ready to move up after two to three years at an agency. Before that happens, account managers are often rotated from one account to another.
What skills you'll need. Excellent writing skills and presentation skills are essential. So are the ability to manage mulitiple projects and timelines. The ability to negotiate well is also a critical skill, since you'll be dealing with agency and client attorneys to insure marketing meets legal requirements, as well as acting as the go-between for the agency team and the client's team. Account management is also a bit of a sales job, as you'll need to sell marketing ideas to clients, and support your ideas as they are developed into campaigns.
How should you prepare? In school, student should take a lot of business courses: general marketing, finance and accounting. As one advances, one gets closer to looking at the numbers that matter to the client. To form an effective partnership, it helps to be able to understand not just their advertising but their entire business. You don't have to be an advertising major, and your previous jobs up until now don't necessarily have to be in advertising. Jobs like being a nanny or waitress use skills that can translate into account management, such as managing different types of personalities and the ability to think on your feet, both key traits for entry level account management.