Be between 18 and 24 years old, blisteringly smart and possessing excellent study habits. Complete your undergraduate degree by the time your Rhodes tenure would begin. The scholarship is theoretically available to students in all fields, but to continue your education at Oxford you must be in a course of study that the university offers (double check at www.ox.ac.uk).
Apply to the Rhodes Scholarship Trust (rhodesscholar.org). Your primary contact will be the Rhodes representative at your university, and this is your best source for application materials. Check with your academic adviser for the name of this person. Meet with him or her as early in your academic career as possible so that you can discuss the selection process.
Review the application. Keep in mind that recommendations from professors are more important than compelling answers to application questions. The selection committee is interested in assessing the impact you've made on your university and the people around you.
Develop effective and mutually beneficial relationships with your professors. You will need complete support from your department, your adviser and any professors you ask for recommendations. Scholarship winners are selected for leadership ability and character as much as for grades.