Advertising is what most usually start out doing, but in the end it's usually word of mouth.
Methods of advertising can include going to local stores and asking if you can put some brochures/business cards out.
You'd also need a fairly mature portfolio as well. Some options are to place ad's in Craigslist stating that you're a photographer looking to build up a portfolio and/or is looking for some models to shoot. Maybe even give the model an 8x10 print of his or her best shot to thank them.
Try to make yourself stand out, and try to get involved with businesses and what not, like corporate portraits for example.
Approach potential clients with a friendly idea/service of portrait work.
Just some ideas.
Good luck!
Well, I'm 13... so would child labor laws apply here?