Portfolios are your visual resumes. It is a showcase for you to present your work and your capability to clients. It is a catalog of your talents, which shows your unique style and ability.
It is important not to confuse, but to assemble the portfolio for maximum effect. A person who looks through the portfolio must not have to ask you many questions. It is just like a well-written storybook. The tale has an introduction, the middle and the ending. By the time the person has gone through the portfolio, he will be able to form a conclusion.
All the presentation materials should move with a planned continuity from concept to concept, and style to style. It must not be just a haphazardly assembled collection of your works.
Some suggestions on making the portfolio:
Faces, because it is the most expressive of all emotions. From the advertising point of view, emotions are what appeals to the audience, whether it is a real life photograph, or a cartoon character.
Subjects involved in identifiable activities like bowling, basketball, swimming, picnic, which are the very activities the audiences are engaged in.
Spot drawings if you are inclined to do this.
Ten to twenty pieces would be sufficient. Not too many to bore, but just enough to show your versatility. Maybe the pieces can be laminated for protection.
Include color if you can do color pieces. One or two pieces should be enough.
Include published work if you have.
Art directors are looking for consistency and versatility. Be selective with your works. It is better to include 8 great pieces than to have 20 mediocre ones.