1. Consider getting some professional shots taken. Professional photography, even if it is expensive, will give you a better idea of what kind of look you give off.
If you've got enough of these photos, consider putting everything into a portfolio, or "book." Bring this portfolio with you to castings or to agencies.
2. Take and know your measurements and stats.
Basic measurements are height, weight, and shoe size.
Know your clothing measurements such as dress size, hip, waist, chest etc.
Know your own personal stats, such as hair color, eye color, skin tone etc.
3. Visit a modeling agency. Almost every major city has multiple modeling agencies, and almost every agency has "open-calls" where they look at new talent.
4.Bring your snapshots and/or portfolio. They will often ask you to walk or pose for them. They may take a head shot or take your measurements as well. If they reject you, don't get disheartened; often an agency is looking for a diverse set of models, so you may just not fit their model lineup right now.