Sight-singing for singers consists of a short piece that will be accompanied by the examiner - both the singer's part and the accompanist's part are printed on the copy.
The examiner will give you the tonic chord, your starting note and a pulse before you get 30 seconds to look at it and will then repeat that information before you start. There are words on the copy as well as notes but I think you can ignore those and sing to "lah" or something similar if you wish.
This is not an easy skill - there are books of examples published. Maybe you could get sight of some if there's time?
Hope it goes OK - remember it's only a small part of the whole exam. Sing well in the prepared stuff, do the aural OK and I'm sure you'll still pass.
Good Luck