First of all, you need to set up a profile with a minimum of 4 portraits. After that, there are basically 4 ways to contact a model.
1) direct contact: search the database to find the model you want to work with. Send him/her a direct message.
2) post casting calls - you could let all model know you want to do shooting and see if anyone answers
3) post availability note: Tell everyone you are available when and where, list a casting call
4) answer casting call for photographers.
At the end of the day, it is about how well you could set up your port and how good is your communication skill. For those just start, you might have to pay some models to shoot in order to build your port. Once you have 7 ~ 8 nice pictures, the models will rush to answer your calls for TF (time for) sessions.
I always make it very clear that the model must sign a model release so I could use the photo legally. The model, in return, will get a dvd of all the images with up to 5 edited.