how do i sing on pitch, melody, and how do i fix singing nasaly and flat?
You can fix singing nasaly and flat by singing with a slightly dopey tone. In addition, being as relaxed as you can when you sing is crucial for a balanced sound. You can find more information at the become-a-singing-master.com website.
Go to a voice coach
Here's a singing lesson download that has exercises and techniques to seriously improve the quality of your voice: http://www.galtmusic.com/lessons/products.html