Learn how your voice works. Your singing voice is an instrument, and understanding how it works, how to use it, and its limitations are all important to you as a singer. A good beginner's guidebook to singing should cover this information. You can also find such a guide for free online (see Resources).
Purchase a reliable singing course that contains CDs and instruction books. These courses are structured much the same as a professional vocal coach would structure a class. These courses are designed to help you get the most out of your voice.
Sing scales on a regular basis. Scales help train your ear, strengthen your range and define your vocal technique. Your vocal course will come with scales for you to sing.
Listen to yourself. It's hard to determine the sound of your own voice when you're actually singing. Listening to a recording of yourself will help you isolate potential problems with pitch and diction. For example, record yourself singing along with karaoke.
Take care of your voice. Sing properly, sing regularly and avoid bad habits such as smoking, which can have an ill effect on your singing voice.