Study the score several times before attending the first rehearsal. You will notice how many times throughout the sections there is repetition, especially in the lyrics.
Listen to the sections you are performing on CD while going through the score. Pick out your part as the singers sing.
Begin singing the section you are working on with the CD as you listen. Sing softly at first so as to be always aware of the other section's entrances. This will help you come in on time every time.
Look for clues to first notes by listening to what the other parts end on. Often you will find your beginning note is the last note sung by another section, or it is in the chord somewhere.
Don't worry about enunciation during those long runs Handel is famous for. Ex: The word "revealed" in "And the Glory of the Lord" and "healed" in "And With His Stripes we are Healed" stretches out a long time and will lose its shape over time but sound the same from the audiences perspective.
Pay attention to your own line of music. Other sections will be going all over the place, so ignoring them except for entrances and cut offs will help you stay on track.
Count constantly. If you are counting you will come in and out on time. If you are not counting, the other sections will go on without you and it will take a few measures for you to figure out where you are.