singing

u2, welcome to A2K. Singing is a matter of breathing and practicing scales. What you refer to as "tremble" is really a vibrato and has more to do with exhaling than anything else. I am surprised that your choir director hasn't put you through the practice routine. If I can help any further, just let me know.

thanks for your help and the welcome. i would just like to know is vibrato a good thing or bad thing and how do you know when you do it or not? thank you again

Well, U2, A vibrato can be difficult when singing in a choir because it doesn't blend as well with the other members; therefore, it may make the intonation (hitting a note on the head and in tune) go awry. Just practice until you achieve the type voice that blends and pleases you, and has an overall good result.