Who you are, plus a credential. Your name and something that differentiates you from your peers (major/degree, athlete, veteran) and/or establishes a relationship (graduate of same college, from the same home town, etc.).
Your specific goal/career interest. This will allow that person to help you or possibly connect you to someone who can.
How you have demonstrated your interest. Demonstrate your interest and experience in the field with examples of things you have already completed. Don't just say "I have always wanted be a doctor," but rather "I have taken pre-med courses and volunteered at the hospital".
Why you are qualified. Demonstrate your qualifications by sharing leadership and work experience, achievements, expertise, skills and strengths.
A question or request for assistance. Consider giving the person two options for ways they may be of assistance. For example, "If your company offers internships, I would appreciate the name of the person in charge of that program, or perhaps I could meet with you in person to find out more about your organization and opportunities in the marketing profession." Be sure to offer each contact your business card for future reference.