You definitely need a portfolio, a great one! I think the best way to create one is by going here. At model mayhem you get the opportunity to work with photographers looking to build up there portfolios who are willing to take free photos in exchange... cool right? There are also wardrobe stylist, makeup artist, and photo retouchers all willing to help you so they can in turn help themselves.
Next you will need to book shoots that do pay. Start of small by going to local business and asking if you could model their products for there promotional posters and such. Then once you've done a few of these you should be good enough to impress an agent.
I hope this helped. Click here to check out my model mayhem page.