If you are involved in theater, you may want to ask around to see if your peers recommend a particularly skilled coach. Usually people in the business have the inside scoop. If you are not in the business, contact your local high school music department to see if the music or choir teacher has any recommendations.
If you are new to singing, one of the first things you will have to focus on is your breathing. Untrained singers typically try to sing from their chests, while trained singers pull air from their stomachs. Your voice coach should give you some different exercises to help you breathe from your stomach. Placing your hand across you lower abdomen should help you focus on this as well.
Your coach will give you a number of exercises to expand the range of your voice. These will likely comprise of repeating particular sounds depending on your needs. Practice these exercises daily.
Sing your assigned songs. Once you start to master the voice exercises, your coach will start to work on particular songs with you. Practice these songs. Your coach should be easily able to detect improvement in your voice as an indicator that you have been practicing, which should motivate you.
Just as an athlete needs to take care of his body for optimal performance, a singer needs to take care of her voice. Eat a healthy diet and get plenty of rest so you are at optimum health. Avoid milk products before singing as they can cause mucus buildup in your throat.