Sing early and often – practice good vocal technique and confident singing nearly every day. Begin looking at the piece(s) you need to prepare early. Make a routine out of practicing sight singing.
Practice your scales – as silly as it sounds, even just singing scales and warm-ups for a while outside of class will help you hear music more effectively. Practice intervals in isolation (do-re, do-mi, do-fa, etc.). Try singing our thirds exercise (do-mi, re-fa, etc.).
Brush up on your music theory skills – https://www.teoria.com/ and http://musictheory.net are two great free sites. Study your theory handouts – if you don’t understand something, ask.
Practice sight-singing – Pay attention each time we sight-sing in class; try to be confident and lead your section when possible. Don’t follow. There are online resources that may help. If you take voice lessons, ask your voice lesson teacher to give you some individual sight-singing practice. Try switching voice parts on a sight-reading (perhaps not during class). I remember switching voice parts for each verse of a hymn when I was a kid.
Don’t forget about rhythm – look for on-line rhythm practice resources, too. Try singing with a metronome to be certain you are keeping a steady beat.
Ear training – in addition to the items mentioned above, study the intervals, scales, chords, and other ear training we do in your classes. Musictheory.net has a lot of a ear training practice material.
Here is some practice material to get you started:
Practice, practice, practice!
Just a little bit of focused practice each day will make a huge difference in your abilities and confidence!