Find out what your range actually is. Starting at middle C on a piano or keyboard, sing the notes up the scale until you are uncomfortable. The last comfortable note is the limit of your upper range. Then find your lower range by starting at C and traveling down the keyboard in the same manner.
Spend time warming up your voice to improve your top singing range. Focus on improving the range that you have, not trying to acquire someone else's. Remember, straining your voice beyond its natural ability can result in temporary or permanent damage to your vocal cords.
Start by singing the word "yellow" in the key of G, working the voice down by "do so mi do" or "8-5-3-1" (It should sound like yel-el-el-ow). This exercise will help you achieve full, broad notes in your upper range.
Continue up the scale using the word "yellow" until you feel the sound vibrate in your nasal cavity or until you detect a nasal tone as you sing. Don't sing any higher than is comfortable for your voice.
Know where your trouble spots are, often called register breaks. These occur when you go from the lower register to the higher register of your voice.