You have identified a list of organizations or companies that you think do interesting work. The first step is usually writing a cover letter and a resume, which will hopefully lead to an interview.
Most people start by working on their resumes.
Samples and information on putting together a resume can be found in Chapter 6 of The Harvard Guide to Careers . One of the key things to remember is that a resume is a selection of information that you choose to communicate about yourself. It is not a definitive, comprehensive listing of everything you've ever done in your life. They are held about twice a month throughout the academic year. See the current Group Meetings Calendar or the OCS Newsletter for details.