I was a model for 4 years, and was accepted by an agency just by walking in off the street and telling them what I wanted to do! A reputable agency might provide you with a portfolio for nothing - as mine did - but beware of places that ask for money from you. By looking in your local telephone directory, you can find the addresses of agencies, and then it's up to you to contact them. A plain head and shoulders shot of yourself is often all that's needed if you wish to include a photo with your initial application, and remember, that on location shoots, you're allowed to be chaperoned. Good luck!