Portfolio is a set of approximately 10 professional photographs that show your best look and diversity.
Before going for a shoot make sure that you understand camera facing techniques, poise and posture, expressions and other modeling techniques. If you are considering a school pick carefully. There are many scams. Check references. Talk to commercial photographers.
At the beginning of your modeling career, your portfolio should include: At least two head shots, two fashion shots, one full-length shot showing the proportion of your body (feet to knees, knees to thighs, thighs to waist, waist to shoulders, shoulders to the top of your head). You can wear anything that makes you feel comfortable and flatters your figure. Remember that the portfolio is about you. Don't let the photographers desire to show extreme lighting or propping take away from your presentation as a professional model.
A portfolio consists of about ten 9"x12" photos. Every portfolio needs a mix of commercial and editorial shots, location and studio. You will also need at least one great body shot and one B&W head shot with minimal makeup.