Take ads you don't like and make them better. And be prepared to tell why you think they are better. Take ad campaigns you do like, and show how they could develop over the next few years. A campaign consists of two or three media, so we're talking about a combination of print, TV, radio, transit, and outdoor. If you know an Art Director or Artist, coerce them into helping you with your layouts.
Nothing is more boring than to see a binder full of words.
When you leave these institutions, you'll have a portfolio to get you on your way. Any training you can get in a visual arts studio or with a magazine, newspaper or department store is very helpful. It could also be worth your while to do some ads or posters for some small retailers. These would augment your portfolio. How To Get An Art Director's Job Read the section for Copywriters.