It's all about the rhythm. Directing a choir is much like directing an orchestra. Choir members will look to you to keep time steadily, give cues when sections are to start in, and to indicate changes in direction and tone. Grand gestures are the norm as being shy will never work here. Be over the top and keep the time, and you will have a successful performance every time.
Learn every part of the piece of music before attempting to conduct it. Practice conducting in front of a mirror. Run through the entire piece, emphasizing each part in turn.
Use a metronome to make sure that the beat is at the correct speed. You may not need the metronome by the time performances come around, but you should use one in the beginning rehearsals.
Check your time signature. Conducting patterns depend on whether a piece of music is written in 3/4 time, 2/4 time, 4/4 time or any others. Know which signature you are conducting so that you will know which pattern to use.
Begin each section with an oversized downbeat. Conduct in the appropriate pattern for your time signature, making sure that each downbeat is a larger gesture than the rest of the pattern so that choir members can keep the beat in their heads.
Use a large pointing gesture to each section of the choir when it joins in the piece. Point and gesture to signify any signature phrases or variations to be sung. Use large arm motions to give cues to each section or soloist as reminders of their parts and places in the music.
Signify long and drawn-out phrases or short choppy stops by using your hands, arms and body language. Hold your hand up to keep the music going, make a chopping downward motion to cut the sound off. Make your body mirror what you want to hear from the choir.
Watching other choirmasters is good practice, as is watching orchestra leaders. They have the same concepts in conducting.