How to remove the nasal , flat sound in my singing ?

One thing a vocal coach will have you do is deep breathing exercises. This is to help you start to generate your sound more deeply centered and to sing from your diaphragm. When you do this, it helps remove the nasal sound. For singing on pitch, it's just a matter of practice and listening. Think of your voice as an instrument and practice tuning it, just as you might a cello or guitar. Sing scales and exercises every day for a warm up. Keep practicing and you will get there.