There is no better time to start building your resume than NOW! But writing your first resume can be tough; trying to focus on your educational and extracurricular activities and less on your lack of work experience.
To get started, I would highly suggest getting a pen and paper and writing a draft/list that you can use as a blue print for your resume. In the draft list all of your educational experience (years, degrees, honors, etc.), extracurricular and volunteer experience (years, description, position), and any work experience, including any internships (years, description, position). Once you have all of this information, you can easily put it into a resume format.