While preparation for singing in the choir is important, maintaining the proper posture while singing is also important.
To help develop good posture for singing, practice this exercise: Place your fingertips on the crown of your head (elbows out). Notice how your ribcage is raised. Slowly open your arms and continue in a downward arc until they rest at your sides. Try to maintain the raised rib cage as you lower your arms.
An expanded rib cage increases breath capacity and provides the ability for air to flow steadily through the vocal chords, which in turn will help your pitch and tone.
But at the same time, allow your neck and upper torso to remain flexible. You should be able to "say no" as you take your breath and begin singing.
One last exercise:
Raise your arms overhead while inhaling; then exhale your air on a "zzz: while slowly lowering your arms to their original position – maintaining a raised rib cage throughout.