The first thing you're going to need to do is decide what kind of portfolio you need to make. Are you interested in web design, print design, product package design, or any one of the many other options out there? What industry do you want to work in? This will help you decide what material is appropriate and what is extraneous. If you're looking for a job in graphic design, no matter how well done your comic art is, it doesn't belong in your portfolio. Logo designers shouldn't confuse the issue by including unrelated work. Keep your design portfolio varied, so it'll show the range of what you can do, but make sure that each item included in it makes sense.
Now, think about the format in which you want your portfolio. The traditional option is a printed portfolio, but these are expensive and are now falling out of favor. Many people now prefer making a CD or DBD or simply printing a few sample pieces with a link to a web portfolio. Online portfolios are becoming particularly popular for freelancers working in illustration and web design. Since their work is likely to be used on the web, many people choose to start there when looking for artists. Plus, it makes it easy for an art director to look at your portfolio from any location.
You'll want to make sure that the samples you choose include only your very best work. It's better to have a portfolio that's a little thin than to bulk it up with subpar pieces. It might be tempting to include more, but stay away from it. You don't want to show low-quality work or make your portfolio seem overwhelming. Fifteen pieces is a good maximum for a design portfolio, and many have far fewer than that. Remember that your portfolio should include a variety of subjects and media. Also, you'll want to take pieces that demonstrate all the basic techniques you've learned. You may even wish to include images of the same piece in various stages of completion. Don't assume that the art director will know that you've got the basics under control. Many beginning designers don't.
For a physical portfolio, use color copies or high-quality printouts matted onto a heavy sheet or black card stock. Each piece should be securely attached and professional looking. For web portfolios and discs, make sure that you're using high-quality images that have been properly color adjusted, and be sure you have no dead links: just like a misspelling in a resume, that looks sloppy. Your portfolio needs to be neat, clean, and organized no matter what medium it's done in. Remember that you'll need to keep things up to date. Make sure any portfolio can be easily modified based on the job you're applying for, or when you have new work to add.
If you'll be promoting your design portfolio on the web, use a good-quality website. Include your first and last name in the domain name or in the title of the page, and make sure that it works well. Free pages and similar choices are not cost savers. They'll be looked upon as unprofessional and should be avoided. It's worth it to pay a little for a proper webpage, since free hosting sites frequently use pop-ups, ads for competitors, and other things that will interrupt your design. Unless you specialize in web design or animation, limit your use of video and Flash on the site. It can look unprofessional and messy, and may crash some computers. You want your prospective employer to have no trouble seeing your work!
A quality design portfolio is extremely important if you want to be able to get the job you want. Whether you work in logo design, web design, print design, or any other field, you're going to need to be able to demonstrate your abilities to prospective employers. A professional-looking portfolio containing your very best work is the way to do it. Make sure that your portfolio is one that'll stand out from the crowd and show prospective employers that you're the right one for the job. You'll have a much easier time if yours is a portfolio that will help you avoid the competition and will show that you have the skills and talents that are needed to be a great designer.