Congratulations!
I'm not familiar with the two pieces, but I would not feel silly at all practicing my part at home by picking it out on the piano. I would want to ensure, by any means, that I am as prepared as I can be for the next rehearsal. Remember:
Practice is for learning
your part.
Rehearsal is for learning how the other parts fit with yours.
Do you know how to read music notation? If you do, and you have a piano at home, you have a good start to learning how to sight-read intervals and rhythms. I entered "vocal sight reading exercises" into Google and got results that may help you.
If you're familiar with intervals and how they look in music notation, play them on the piano and create mnemonic devices by coming up with songs for each example (i.e. "Here Comes the Bride" for perfect 4th).
If you're not familiar, start with your choral parts and work on identifying the intervals. Make sure you know what a major and minor triad look and sound like and you can use that as a reference to find many intervals. Beware the tri-tone!


