Breathe correctly using your diaphragm. Lie down on a bed or floor and breathe naturally. Notice as you inhale that your stomach rises, and, as you exhale, it flattens. This natural breathing is what you should aim for when you speak and sing, being careful to take deep breaths as you run out of air. Practice speaking relatively slowly so that you are aware of bad habits such as cramming too many words into one breath. Notice there is no pressure or tension held in any part of your body other than your stomach/diaphragm.
Repeat the phrase "Um hmm," as if someone asked you a question to which your answer would be yes and you answered, "Um hmm." The noise you make doing so is your most natural voice. Notice how the sound resonates in the front part of your mouth and even vibrates your teeth. This is the healthiest resonance for your voice. Hold the "m" at the end to feel the vibration further. Notice that your pitch is most likely higher than you are used to speaking.
Chant the following forward-sounding phrases: "moon and mars," "Minnie marries Mickey," "mother makes muffins." You should feel the vibrations heaviest on the "m" part of the phrases. Now copy the same feeling, repeating the phrases in a sing-song manner, with elongated words. Practice this exercise several times a day, always focusing on keeping you voice forward instead of back in your throat.
Continue practicing the above vocal exercises and add the following phrases to your chants/phrases: "all the girls were laughing," "my movie made Tim and Tom sad," "come in and close the door." Practice these several times a day. When you speak throughout the day, copy the feeling you get focusing your voice forward in these phrases.
Apply the same forward placement when you sing. Sing the phrases listed in steps 2 and 3, on one note at a time moving up the scale, keeping your voice focused forward. After a few weeks of practicing and being conscious of moving your voice to a forward place of resonance, you should have much more strength and power to your voice, which you should be able to apply to your singing voice. Celine Dion places her voice forward and, in doing so, achieves brilliant sound quality. By simply changing your voice placement, your range should improve as well. Enjoy your new found natural voice.