I am sure that you already have great possibilities for your singing voice if people are already telling you that you have a great voice in their opinion, but...
The answer above is correct as far as singing from your diaphragm is concerned. The diaphragm is the muscle wall between the chest and the abdomen that the body uses for breathing. Your choir teacher will be able to explain it's function to you as how it is used in singing. Additionally, think of what your stomach area does when you are laughing really well. Your stomach area should have that same sort of feeling when you are singing. It should be moving in and out.
As far as using your whole body in singing, that is correct also, that you should used your whole body. Your posture when you are standing or sitting should be really good so that the air can flow really easilly. In singing, your face is called the "mask." The whole structure of your face, bones, skins, cartilage, etc. come into play as far as your voice tone. Also, your neck and chest area. These areas need to be very open and relaxed. Again, these are things that your choir teacher can explain to you.
This is a very complicated subject, and one that is difficult to address in just a single answer. I have been involved in all facets of the musc industry, both instrumental and vocal, professionally for over 36 years.
I am going to try to make it simple right now for you, so that you can at least get started on your own.
1. Try to imitate the sounds of people's voices who can really sing and utilize their voice to it's fullest potential. Those would be people that sing in the operatic style. It doesn't mean that you have to sing opera. But, just try to imitate the sound of a person's voice who sings opera. It is very hard to find anyone who truly utilizes their voice to it's fullest potential who sings any sort of music that is really popular with most of the people in a culture - rock, jazz, country, etc. I'm not saying that their music isn't good. It's just that the majority of them don't truly utilize their vocal instruments. As a matter of fact, some of those styles of music wouldn't sound as good if they did change the sound of their voices.
But, you are wanting to change the sound of your singing voice so that it doesn't sound so nasal. And, I am trying show you how to do that by starting on your own.
2. When you sing, take a deep breath and make your stomach really hard. Try to push the air out of your voice with the muscles around your stomach. Open your mouth really wide and try to have the feeling in the back of your throat like it is when you are yawning or going to the doctor and he or she says, "Say, ah." so that they can check the condition of your throat.
3. See if you can take voice lessons privately from someone.
Additionally, you might find some things on the following sites helpful to you.
How To Improve Your Singing Voice Instantly - Learn To Sing Better!
How to Sing - wikiHow
Vocalist - Singing and Singing Related Articles
Vocal Exercises to Improve your Singing Ability: Free video clips on ExpertVillage.com
Vocal Exercises to Improve Your Singing Voice
Dummies::Breathing Basics for Singing