The hardest part of creating a portfolio can be to take good pictures of your work. This site has some tips about shooting your work and then goes over some Photoshop methods of cleaning up the images.
Sometimes the biggest favor you can do for yourself is get a professional to look over your portfolio and tell you what they think. National Portfolio Day exists for this exact purpose--you can meet with representatives from colleges (there are days for undergraduate and graduate students in art and architecture).
The format, size, and style of your portfolio will probably depend greatly on where you're planning to submit it. Don't send the same CD-ROM to every place you apply whether it's an art school or a gallery. Even art schools all have different requirements for portfolios, so do your homework beforehand.