Do your research. Explore the websites and try to choose what looks to be a reputable agency. One that does not promise too much but which demonstrates a high calibre of clients and a roster of the kind of models to whom you aspire.
You must then decide how best to make your approach.
Most agencies have a walk-in where you arrive at their door and will be seen by one of the bookers. If you bring any photographs they do not need to be professional. Holiday snaps are perfect because, should a reputable agency be interested in you, they will first arrange for professional, test photography to be taken and it is this that will determine whether or not they take you on. How you look in front of the camera will be the final arbiter! You will never be charged for tests unless you are taken on and then, never before you have earned sufficient money.
Be prepared to wait because most walk-ins are during half terms and holidays. Also, be prepared to be turned down because, whilst your face may be perfect to your friends and family, it may simply not be suitable for a particular agency. Usually, you will be referred to other agencies who may be better suited to your particular look.