1) Ask For Recommendations
If you see someone with a great style, ask them about it! They'll be thrilled that you asked.
2) Bring Pictures!
Do your research! Buy a couple of beauty magazines like Allure, Cosmopolitan and Marie Claire, or find pictures of celebrities. Bring in as many pictures as possible of the style that you want.
3) Know Your Hair!
Stylists are not magicians. Is your hair thick or thin, fine or curly, wavy or straight, flat or frizzy? These details are important for your hair cut. If you want a certain hairstyle, it may not be right for your hair type - only your hairdresser can know for sure.