Good genetics is key. If you have legs that you think are model-worthy, approach the usual advertising agencies / model agencies. This is all basic, common sense - It seems like you do not have the initiative to make it far IMHO.
For a starter advertise yourself in one of the forum (not necessary to be Clubsnap), but clubsnap is a good place to start off (Modelling Services Directory).
You can do it for free, just to build up your confidence and portfolio. When you have a big enough or good enough portfolio, then you can approach advertising agencies or modeling agencies and ask if they need new model (be sure to show them your portfolio).