Mostly singers do proper warm up before they start to sing. Proper warm up exercise before singing is necessary. Why? Simply because it help you to open up your vocal chords and avoid damage to the delicate vocal nodes due to the high notes. Starting with a good warm up exercise will improve your vocal range and make it easier for you to hit the high and low notes.
This could be a big help for you on singing on key. Remember do not forget some necessary reminders if you are really interested and serious on your singing profession. It may badly need much of your patience and deep understanding.