If you have sheet music for choir, it usually shows you the parts for all four voice sections and for the piano accompaniment. If you have an entrance where you have to hit a certain note and you can't find it in your head, then look through the notes in the two or three bars before your note -- look in the other parts on the other staves. It may be that the piano is playing that particular note a couple of beats before you need to sing it. Or perhaps the altos or tenors are singing that note (in a different octave). If you can find it, then circle your note on your staff, and circle the same note that occurs earlier on another staff, and draw a line between the two.
Then the next time you are rehearsing that section, you can remind yourself to listen for that note to be played or sung by the other parts, fix that note in your head, and then you will be prepared to sing it.
Any time you come across a tricky section where you are making mistakes in your own part, circle it in pencil. Then during breaks, look for other parts in the sheet music that can help you figure out how to sing that particular problematic lick in your part. Make as many annotations (lightly in pencil) on your sheet music as you need.
Also, in rehearsal, during breaks in the action and when you are sure that you are not being disruptive and it is not interfering with the work of the choir director, you can lean over to your fellow musicians, point out a problem area you have circled, and ask them to explain or sing or hum the lick that is giving you trouble. Be careful, though, because a choir contains a lot of people sitting close together, and singing or humming out loud during breaks can be disruptive to others' concentration and thought processes.
Update: Each note depends upon a different context in the music. Finding the note you need to sing is always dependent upon the notes that the other vocal parts are singing at that instant, and the notes that the piano accompaniment is playing. The first time you have a particular note to sing, it might be one note in a chord where your note fits neatly into consonant intervals being sung by the other voices, so it's easy for your ear and your brain to find your note. The next time this note appears, it may be in a dissonant interval against one or more of the other voice parts. That would make it harder for you to sing it, because it might sound "wrong" to your ears even though that's the way the composer wrote the song. Now you don't need to analyze all the chords in detail -- that takes time, schooling and ability. But you can learn to become aware, in general terms, of consonance and dissonance at any given point. Your choir director might point out a certain note and say, "Hey, Basses, that particular note is supposed to clash with the tenor note. Don't be shy about singing it", etc.