Determine if your nasal tone is the result of passing too much air through your nose and cutting off the flow through your mouth. This phenomenon is often habitual and occurs in the speaking voice as well. First sing "ee-aah-ay" as you normally would. Sing it a second time with your nose pinched shut, and if your voice gets blocked off then you're singing through your nose.
Find your soft palate by feeling the roof of your mouth with your tongue and then moving your tongue back toward your throat until the roof of your mouth feels softer. This is the difference between your hard and soft palate. Lifting the soft palate opens up space for air to pass through your mouth.
Keep your mouth open while singing. Otherwise, less air and volume can escape. To help get a feeling for an open mouth while singing, watch yourself in a mirror and see if it's possible to fit a finger between your teeth as you sing a song.
Become familiar with the feeling of an open soft palate. Yawn and try to sing "uuuh" while yawning, which should make the soft palate rise. The voice this creates is not a good singing voice, but it is an excellent exercise for learning to create more resonant space through the mouth.
Sing with an exaggerated tone that lowers the back of the tongue and opens up the mouth cavity for air to flow through.
Relax and don't force your singing. Singing is a natural act and becomes more difficult the more tense the singer is. To reduce forced singing, some people find that it helps to stop singing and bend over at the waist, letting the feeling of gravity reduce the tension.