You've got to show them examples of your work that make them want you to produce similar work with them. In other words, you need a portfolio that demonstrates you can make it worth their time and effort.

Of course that poses the question, "How can I build a portfolio if no one will work with me?" You have to start small. Check among your friends or family to see if anyone is willing to let you photograph them. You don't necessarily need to start right off with photographing those you know well for nude sessions. But doing portrait work, especially that is creative and demonstrates your ability to visualize a setting, control the light in the scene, and compose the shot will go a long way to show that you are serious about doing good work and not just another guy with a camera that wants to take snapshots of young, attractive women in various states of undress.

As other answers have suggested, you might also look into taking a class that would provide the opportunity to work with models in a group setting. This would give you the opportunity to build and improve your portfolio. It might also give you a chance to meet and become acquainted with models that could potentially be more comfortable working alone with you in the future after having been around you a bit.