Always run through a few exercises before you start singing to warm up your vocal cords. Singing scales is a traditional vocal warm-up exercise, but many other types of exercises exist. Try holding your cheeks up with your hands, letting your lips get loose and blowing. Let your lips flop around as you make a horselike sound. Try singing to your favorite songs using this technique. After a while, this exercise will actually help open your high register and allow you to reach high notes. Search for different types of exercises to help you become a better singer.
Vibrato is a rapid slight pitch modulation. When first learning vibrato, practice alternating slowly -- two to three times a second -- between two notes a half-step apart, like a G and a G#. Pay attention to keeping the vibrato steady. When you can confidently do vocal vibrato at this rate, try speeding it up. Use vocal vibrato in songs that you like to sing to get a feel for adding it at the right places.
Oftentimes, the source of strange vocal tones, such as nasal sounding singing, is from singing too much from one of the three major acoustic cavities in your head: nose, mouth and throat. The ultimate goal is to balance your voice so it is being produced equally from all three of the regions. The way to balance your voice is to stay relaxed when you are singing. Do not attempt to force your voice through any specific cavity -- just let it flow. If you can achieve this balance, then you will find that your vocal tone has changed, generally in a positive manner.
One of the keys to being a great singer is the ability to control your vocal dynamics. In a nutshell, dynamics is the relative loudness or quietness of your vocal performance. By singing quietly when a song calls for it; turning up the volume when a climactic part comes along; then returning to a normal voice, somewhere between loud and quiet, you can significantly strengthen your vocal performances. Listen to the ways your favorite singers use dynamics in their songs, and try to copy their approach.