Record the entire choir singing with a recording device, or purchase a recording for which you have the music. You can often find a professional recording of any piece your choir is singing.
Play your part on the piano along with the recording. If you don't know how to play piano, you must learn, as success as a vocalist requires passing piano skills.
Play your part on piano while singing along with the recording simultaneously. Listen to how your part fits in with the rest of the ensemble.
Sing along with the recording without using the piano. If you experience trouble, go back to using the piano to help isolate your individual pitches.