You'll need a 3 ring binder. Divide into subjects with tabs labeling each of the required subjects for your state. This will vary, but the typical subjects are: math, science, social studies, language, and reading. You will now have a section for each subject.
Add work samples throughout the year to each subject. Be sure to date the work samples when you add them. It helps the reviewer if your samples are in order by date. The reviewer will be looking for progress in each subject throughout the year, so dating is essential.
Here are some good items to add to your portfolio:
spelling tests, stories your child has written, tests from each subject, semester and unit tests, a list of books your child has read, list of field trips, pictures that show your child working on projects and artwork, pictures of finished projects, daily worksheets.